I can't answer all of your questions, but I'll try to give you some thoughts relevant to them.
First, I'd suggest that you get the JTAG box from GlobalScale if you aren't already getting it. You need it to access the UART and JTAG. You only need to hook up the UART to get the serial console and there are some who made their own UART connection (http://plugcomputer.org/plugforum/index.php?topic=2052.0
), but the JTAG is needed to recover from a bricked plug (but you can flash U-Boot over UART if the plug isn't bricked). I'm a software guy so I wouldn't attempt to make my own as I'd probably just end up hurting myself with some wire crimpers. As an aside if you do brick your plug, the GuruPlug Installer used to recover is based on openocd (like the SheevaPlug Installer is) and you have to hook up both UART and JTAG (at least that what I had to do). When trying to recover, I found that you don't have much time once power is applied for it to work; have the command pre-typed, plug in the plug with the connections already in and then rush back to the keyboard and hit enter right away (was suggested here: http://plugcomputer.org/plugforum/index.php?topic=2109.msg12424#msg12424
and worked for me).
The U-Boot and kernel that it comes with have the wrong arch number and won't work anyone but each other (not quite true, if you build the other to use that wrong number, you can get around that...or so I've seen on the boards). If you go to the downloads section of GlobalScale's website, there are links for an updated U-Boot and kernel that use the right numbers (http://www.globalscaletechnologies.com/t-downloads.aspx
). I highly suggest that you upgrade the U-Boot because the version that ships is pretty limited. A build of U-Boot was built by forum member flipflip (available from http://oinkzwurgl.org/guruplug_uboot
) and is worth looking at (also look at http://plugcomputer.org/plugforum/index.php?topic=1642
as it discusses attempts to put the kernel on microSD and external USB, although I don't know of anyone being successful). One other note on booting the kernel: you can tftpboot the kernel and that's a good thing for development when you don't want to commit to the kernel build or to test the build.
As for distros that support the GuruPlug:
Debian Squeeze (doesn't mention GuruPlug but does support Kirkwood): http://www.debian.org/ports/arm/
armedslack, Slackware's ARM port (GuruPlug is one of their supported platforms): http://www.armedslack.org/
FedoraARM, Fedora's ARM port (they provide a link to a blogpost about getting FedoraARM up and running on a GuruPlug): http://fedoraproject.org/wiki/Architectures/ARM
PlugBoxLinux, a derivative of Arch Linux for Kirkwood devices: http://plugapps.com
Gentoo (doesn't specifically mention GuruPlug, but does support armv4 and later): http://www.gentoo.org/
There is an instance of someone getting Angstrom working with the GuruPlug (http://plugcomputer.org/plugforum/index.php?topic=1790.0
), but I've not been able to get that to work. I built it for the SheevaPlug (as the other poster implied they did) but I used the development branch ... and I digress ...
For completeness, I'd be remiss if I didn't mention the wiki pages: http://plugcomputer.org/plugwiki/index.php/GuruPlug
Hope that helps.