As far a I understand you can't go beyond 2.6.28 (which is why KP still is 2.6.28, although heavily tweaked) because some of the non-free components (drivers, probably apps) would cease to work.

The Linux kernel infrastructure (modules) is highly version dependent. Have a look at /lib/modules and you'll see that there is a directory for every kernel!
Linux experiences frequent ABI changes, which means to be on the safe side you'll have to recompile all your kernel modules for every new minor revision (3rd number in 2.x kernels).
For that of course you need the code of these modules. Unfortunately we don't have this code for all the modules in the Maemo kernel, which means we can't move on.

This is why it's so important for the Neo900 to be as free (libre) as possible and identifying (easy) and replacing (hard?) non-free components is an important task for FPTF. In fact being able to use a newer kernel (3.x) is one of their main goals.