ubuntu is a good deal because virtually everybody will be able to add their own stuff with just an apt-get - the down side is boot times.. although with upstart it got a lot better
on my current carpc i have an ubuntu with initng and ratpoison and i can boot in 20s from poweron to xterm - it'd be very good if we could get 15s (that's just a little more than my display needs to unfold)
frontends:
- myth is good, touchscreen support is in, but it isn't that great (still need an escape key, music selection sucks with touchscreen)
- nghost, still in development (kev and i are working on it, you're welcome to join, or comment) - has great touchscreen support and is geared towards being the best carpc frontend for linux - downside: doesn't have a plugin framework (yet) -> a non-issue if you're not a dev, and are happy with what's in (mp3 playback, video playback, rss, gpsdrive support (

) )