Having Ubuntu as an officially supported OS wouldn't hurt for sure.
And it would definitly give you some free PR, but I guess the default option should be something more simple, or better something that can be easily controlled by a dpad and a few buttons.
A window (as is individual windows, not MS Windows) based OS simply isn't really made for such a small screen and no mouse (touchscreen is a good, but not an exact replacement for using such an OS).
Hmmm... however Ubuntu MID seems to have done away with the windows:
http://www.ubuntu.com/products/mobile
Maybe it is not a bad choice after all.
Edit: Hmm Ubuntu is actually working on making it N800 and the like compatible:
http://jonreagan.wordpress.com/2007/07/24/...worths-keynote/
If it runs well on the pandora, I guess it should be made the default "enhanced" user interface.
Edit2: Yeah... all discussed in the Ubuntu MID thread.... my bad
Edit3: Conclusion: if it is doable you should go for it, but maybe add a boot option for simple (gmenu) and advanced (ubuntu MID user interface) option. Only if it actually improves boot time though, otherwise go Ubuntu MID all the way (shutting it down for big emulators and games should still be possible right?).
But yeah I like the idea, and you will not get such a polished GUI otherwise I guess... the only problem is that it will probably not fit into the 256mb NAND right?