That seems to have worked!  I can now access the CUPS admin UI on the plug, from my PC.  Now all I need to do is figure out how to use CUPS!  Thanks to birdman and AutoStatic for their help and suggestions.
...jeff

In the CUPS config file, I set the LogLevel to debug to get more detailed logging, and when I try to open the CUPS UI from a browser in my PC, I get a "403 Forbidden", and the log file on the Plug gets this added:

D [23/Aug/2009:09:45:35 -0500] cupsdAcceptClient: 7 from 192.168.1.102:631 (IPv4)
D [23/Aug/2009:09:45:35 -0500] Report: clients=1
D [23/Aug/2009:09:45:35 -0500] Report: jobs=0
D [23/Aug/2009:09:45:35 -0500] Report: jobs-active=0
D [23/Aug/2009:09:45:35 -0500] Report: printers=1
D [23/Aug/2009:09:45:35 -0500] Report: printers-implicit=0
D [23/Aug/2009:09:45:35 -0500] Report: stringpool-string-count=236
D [23/Aug/2009:09:45:35 -0500] Report: stringpool-alloc-bytes=6152
D [23/Aug/2009:09:45:35 -0500] Report: stringpool-total-bytes=5040
D [23/Aug/2009:09:45:35 -0500] cupsdReadClient: 7 GET /printers HTTP/1.1
D [23/Aug/2009:09:45:35 -0500] cupsdAuthorize: No authentication data provided.
D [23/Aug/2009:09:45:35 -0500] cupsdSendError: 7 code=403 (Forbidden)
D [23/Aug/2009:09:45:35 -0500] cupsdCloseClient: 7
D [23/Aug/2009:09:45:36 -0500] cupsdAcceptClient: 7 from 192.168.1.102:631 (IPv4)
D [23/Aug/2009:09:45:36 -0500] cupsdReadClient: 7 GET /cups.css HTTP/1.1
D [23/Aug/2009:09:45:36 -0500] cupsdAuthorize: No authentication data provided.
D [23/Aug/2009:09:45:36 -0500] cupsdSendError: 7 code=403 (Forbidden)
D [23/Aug/2009:09:45:36 -0500] cupsdCloseClient: 7
D [23/Aug/2009:09:45:36 -0500] cupsdAcceptClient: 7 from 192.168.1.102:631 (IPv4)
D [23/Aug/2009:09:45:36 -0500] cupsdReadClient: 7 GET /favicon.ico HTTP/1.1
D [23/Aug/2009:09:45:36 -0500] cupsdAuthorize: No authentication data provided.
D [23/Aug/2009:09:45:36 -0500] cupsdSendError: 7 code=403 (Forbidden)
D [23/Aug/2009:09:45:36 -0500] cupsdCloseClient: 7
D [23/Aug/2009:09:45:39 -0500] cupsdAcceptClient: 7 from 192.168.1.102:631 (IPv4)
D [23/Aug/2009:09:45:39 -0500] cupsdReadClient: 7 GET /favicon.ico HTTP/1.1
D [23/Aug/2009:09:45:39 -0500] cupsdAuthorize: No authentication data provided.
D [23/Aug/2009:09:45:39 -0500] cupsdSendError: 7 code=403 (Forbidden)
D [23/Aug/2009:09:45:39 -0500] cupsdCloseClient: 7

Looks like I'm connecting now, but seems like I'm not being authenticated as a user. 
Any ideas anyone?  Thanks for all your help.
...jeff

Progress!  But not success yet. 

I edited the /etc/cups/cupsd.conf  file, by changing   Listen localhost:631  to   Listen 0.0.0.0:631 .  I restarted CUPS with "/etc/init.d/cups restart", and now I get "403 Forbidden" instead of "Failed to Connect". 

I don't see anything in the log file (/var/log/cups/error_log) that corresponds to those errors.  But I did notice this in the cupsd.conf file:
# Administrator user group...
SystemGroup lpadmin

I tried changing that to "SystemGroup *"  but the log file says that's not recognized. 

Anybody got any ideas at this point? If I try to bring up the CUPS UI from a computer external to the plug, how do I arrange to be a member of group lpadmin?

Here's the netstat output.  What would this mean?

root@debian:~# netstat -tan | grep LISTEN
tcp        0      0 0.0.0.0:139             0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:445             0.0.0.0:*               LISTEN     
root@debian:~#

My PC is assigned 192.168.1.102, while communicating with wireless router 192.168.1.1.

The wireless router is assigned 192.168.0.100, while connected to 4-port wired router 192.168.0.1.

The wired router is plugged into a cable modem.

The SheevaPlug was plugged into the wired router, and was assigned IP 192.168.0.102. In this configuration I got the "Failed to Connect" errors while trying to connect to the CUPS UI from my PC using:  http//192.168.0.102:631/printers  .

After reading your note I tried plugging the SheevaPlug's ethernet cord into the wireless router, and after reboot it was assigned 192.168.1.103. I still get the "Failed to Connect" errors.

I guess I need to take a networking course... 
Thanks for all your help and suggestions!
...jeff

on the plug:

root@debian:~# iptables -L
-bash: iptables: command not found

on my PC:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

When I try the telnet:

Trying 192.168.0.102...
telnet: Unable to connect to remote host: Connection refused

Like I said, I have major holes in my knowledge!

How do I check to see if I have a firewall running on the plug? I'm running the Ubuntu server that came already loaded, and I don't think I added a firewall. I fixed the DHCP problem that was mentioned on this forum, installed the NTP time service, ran apt-get update, and not much else.  Could I have done something that enabled the firewall?

My PC is running Ubuntu 9.04 desktop, BTW.

Thank you for your reply, AutoStatic.  I tried connecting to the CUPS web UI with  http://192.168.0.102:631/printers  and got this message back from Firefox:

Failed to Connect
Firefox can't establish a connection to the server at 192.168.0.102:631.
Though the site seems valid, the browser was unable to establish a connection.
    * Could the site be temporarily unavailable? Try again later.
    * Are you unable to browse other sites?  Check the computer's network connection.
    * Is your computer or network protected by a firewall or proxy? Incorrect settings can interfere with Web browsing.

The IP address is correct because I'm ssh'd into the plug with that IP.
I also tried  http://192.168.0.102:631  with same result.
Would this be a problem with a port that needs to be opened in a firewall, either on my PC or on the plug?  These questions make it painfully obvious where the holes in my knowledge are, but I really appreciate any assistance the group can give me. I downloaded the CUPS manuals and looked them over, but that doesn't help much unless I can connect to CUPS!

I'm bumping this up to try again to see if anyone has instructions or suggestions for how to set up a basic network printer on my home network.  I'd appreciate any advice or comments!
Thanks!
...jeff

I'm curious: why are there no air holes in the top of the plug case?  When the plug is in its normal orientation (plugged into a wall socket, ethernet and USB connectors pointing straight down) there are air holes on both sides of the case at the bottom, but NONE at the top. Wouldn't it make more sense to have some holes at the top, so the hot air inside would rise out the top from convection, and room temperature air would come in at the bottom?

I have an HP deskjet printer plugged into the USB port on my shiny new sheevaplug, and I installed CUPS using apt-get, but now I'm not sure how to proceed.  I would like to make this printer available to all the Linux and Windows computers used here in my house (all running through the same routers).  Has anyone done this, and is there a how-to already posted that I'm missing?  (BTW, I also plan to add a USB disk drive for my family to have access to.)