i setup an XBMC media center on my PC, with the "trancparency!" skin it looks amazing, and the first thing that came to my mind was that it should be used as a frontend in mobile computing.
but i guess people dont use it often because of the following reasons
1.) i dont think its really touchscreen friendly
the only reason i started my carpc project is XBMC (its still not complete) but the greatest problem in my build is also XBMC since it needs so many keyboard commands to do stuff, so ive been making lists of most used buttons, thinking of how to integrate so many buttons on my dash, im looking into steering wheel button control, adding a bunch of buttons under/side of touchscreen, even looked into Media center remotes, even ipod touch with XBMC remote app.... still not finalized anything!!!
2.) lack of ability
most front ends used on here treat multimedia as a secondary thing, and are better at doing other stuff, like navigation, real time info of car performance data , OBD etc, browsing internet, and stuff like that, whereas XBMC can only play audio and videos as far as i can tell
3.) most people who use XBMC, dont put so much effort into their music library, to sort it, name it , tag it, scrape it, because unless u do that, u dont really see something beautiful...and the skin needs to be "Trancparency!"
BUT as far as im concerned, im making my car PC ONLY for using XBMC inside my car so i will come up with solutions, and compromise where need be... so good luck to u and me!!