Seems to be a common prob. I guess the Fedora default filesystem layout does not take into account that anyone may actually want to upgrade using preupgrade, or I suppose any other method other than a clean install.
On my FC11 machine as noted in earlier posts deleting everything not required to boot gives just enough space to preupgrade. Eventually worked fine.
My poor FC10 machine didn't have a chance, /boot was smaller there.
I got round it with lots of google'ing to figure out how to use resize2fs, lvresize, pvresize, parted to zap the /dev/sdaN to a smaller size etc, then using clonezilla to back up the smaller partition assigned to LVM.
Then deleting all LVMs, using parted and resize2fs to enlarge /boot and create a new /dev/sdaN to restore into, and restoring the smaller LVM partition backed up by clonezilla. Then mucking about to grow the restored LVM pv/vg/lv to use what was left again.
Lots of hassle. Went and redid the exercise on the FC11->FC12 machine where preupgrade worked as well to avoid probs when FC13,14,15... appears. All my /boot filesystems are 2Gb+ now.
It was a lot of hassle; don't know why the default filesystem layouts make /boot too small to use preupgrade, but if the defaults were a decent size to start with nobody would have probs using preupgrade.
Would be nice if the default filesystem layout on fresh installs didn't use the smallest space needed for /boot for that Fedora version.
I guess going forward if anyone knows anybody about to start a fresh install just shout at them not to use the defaults for filesystem layout and make /boot at least a couple of GB.