Wrong. Alternate kernels have been compiled. I'm not certain whether the approach Nokia uses involves binary lumps that are linked in, or entire binary-only modules, but either way, it works, as long as you use the same (or close enough that no relevant interfaces have changed) kernel version. Even if not, you could always use the whole Nokia kernel.

Yes, it's quite possible. But Ubuntu can't distribute those (and may not want to, for ideological reasons), so there's some serious hurdle-jumping required to get a complete installation. Take something from Ubuntu, add some Nokia stuff to make an image on the computer, then flash it or load it on an MMC partition. Check out the current progress with running Debian Sid; and that's already compiled for armel. Here you have to compile everything, and then mix and squeeze it into the tablet.