If the OP doesn't have the Nvidia proprietary drivers installed, blacklisting nouveau would not be a good idea. If he/she does have the drivers installed nouveau would already be blacklisted. Since F11 is using the 2.6.30 kernel and F12 is using 2.6.32 kernel a complete reinstall of the Nvidia drivers might be needed, if in fact they were installed in the first place. Then again, I don't use preupgrade to do upgrades, and "smoker" may be correct if preupgrade didn't carry the kernel options into the new version.
Confused? So am I. I guess the question should be if the OP is using nouveau or proprietary drivers. The default /boot size for F11 was 200mb, preupgrade was having problems with space on the /boot partition for the longest time, and I wonder if that could also be the problem. Were extra kernels etc. cleaned up out of the /boot partition before using preupgrade?
Checking through common problems using preupgrade in F12 might help: