As for the Linux kernel, Dalvik (and sometimes Launcher, depending on the device) which comprise Android itself, the source code is there and ready for them and it's up to each device manufacturer to gussy it up like a whore before releasing it to the audience for a song and dance. In most cases, they're slow to roll out because they're being meticulous with their code review but I'm betting that as time goes on, there'll be less and less gussying up by carriers because it both ruins the experience for users and also causes the carriers to spend lots of time and money to customize. Vote with your dollars, and you'll see more carriers doing whatever brings in the cash more often. (Which is why I didn't buy back into Maemo this time around... I'm still waiting to see if Nokia improves things or just repeats their mistakes.)