It's in the documentation package, titled USB Flash Recovery from U-boot.  The process is essentially the same as flashing from NFS, but without all the hassle of setting up an NFS server.

You may want to follow my thread on problems with it over in the U-boot section of the forum before you try it yourself.  I spent about four hours last night trying to flash from USB, and end up with nothing but a kernel panic.  I've followed all their documentation, other hints from around the net, and a few of my own ideas.  Not sure if it's specific to my device, or if it's a general problem.

Thanks for the reply.

It is failing after I have written everything, and am trying to boot from the internal NAND.  I flashed using loadaddr 0x2000000, mtd1Size 0x300000, and filesize 0x44ad000.  All those arguments I got straight from the documentation (USB Flash Recovery from U-Boot-Rev1.2.pdf).  Are there different values I should try?

I just tried booting with the bootargs you suggest (which I believe are the same as in the documentation).  Still nothing.

I went through the entire process again (except for re-upgrading u-boot) and saved a log of it, which you can download here:
http://djpandemonium.com/sheevaflash.txt

Has anyone successfully flashed their device from USB?

I follow the v1.2 instructions in their documentation.  I download and verify all the files from Marvell, and flash u-boot-rd88f6281Sheevaplug_400db_nand.bin from a tftp server.  I then follow all their instructions, but when it comes to the final reboot into the supposedly working system, it ends with:
---
Root-NFS: No NFS server available, giving up.
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "mtdblock2" or unknown-block(2,0)
Please append a correct "root=" boot option; here are the available partitions:
1f00       3072 mtdblock0 (driver?)
1f01     520192 mtdblock1 (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)
---

I tried switching root=/dev/mtdblock1 just for kicks, even though I know that's not right.  Of course it doesn't work.

Anyway, I've reflashed it now about six times, redownloaded the files, reformatted my flash drive, and tried all kinds of variations on their instructions.

Any clues?

It contains files for flashing a U-boot image and/or the root file image to the plug via a USB flash drive plugged into the plug's USB port.  The other documented method is by booting (mostly) the same data from a linux host via tftp/nfs.  The USB method is quicker and simpler to set up.

Has anyone discussed customized Sheevaplug runs with Globalscale and/or Marvell?  Are they the ones to talk to for creating an end-user ready sheevaplug product by flashing custom software, applying any physical branding (decals, etc), and packaging? 

I'm certain that there are a number of factors, but has anyone seen a pricing schedule for such work?

Yep.

apt-get install openvpn installs v2.1 rc11.  It's interesting that the stable 2.0 tree isn't what's in the repository, but a release candidate for 2.1.  It's not the latest one (that would be 15), but it's pretty recent.

Someone has already done some speed tests:
http://ubuntuforums.org/showthread.php?t=1082489&page=7

Look at the second and third posts.  It looks like USB speeds are roughly ~22M/sec write, ~32M/sec read.