Files located at the bottom of the post.
Ubuntu runs fairly nice on the GTab. Find a video below to see it boot. Performance is not as smooth as Android, but I've already narrowed down some kernel tweaks to speed it up.
Video (not accelerated)
What doesn't work (yet):
Audio (pulseaudio hangs, haven't investigated)
To get it working, I used a custom kernel with added support for various drivers, but otherwise pretty much the stock VS kernel. It gets the rootfs from the micro SD card, so it only requires a boot.img flash (so it doesn't destroy any user data or the Android system partition).
Youtube video of my gtab booting into Ubuntu
Using linux (via an existing install, or a LiveCD), format a 4GB or larger micro SD card as ext3, and untar the karmic-rootfs on it.
cd /media/sdcard tar -xzvf /home/user/Desktop/karmic-rootfs.tar.gz
Insert the sd card and boot. You will see some yellowish/gold colored text quickly scrolling across the screen - this is normal. This should boot straight to a GDM login screen.
Wireless will work with wicd (not NetworkManager), and you should use a USB hub with a USB keyboard & mouse.
Karmic rootfs tarball (1.1GB)
Karmic kernel update.zip