I looked at this just a bit myself but got lost in python. :) It looks like if I select an ext3->ext4 migration during an upgrade, the filesystem gets properly flagged as test_fs, and the fstab gets set as ext4dev, but the filesystem is still mounted as ext3 when the upgrade begins to install packages. It'll mount as ext4dev when I reboot, but all the new files will be ext3-formatted. This may be because blkid still reports it as ext3? Since there is no extent-based file on the system yet, it would, prior to any new write. I couldn't tell for sure if there was a simple way on migration to specify that it should be mounted as the new (migrated) fs type... Or maybe I'll get tune2fs to set the extents flag so that blkid reports the device as having ext4...
Hmmm, I thought I had fixed this up. Will dig into it, but probably not tonight
Not doing migration to ext4 for F9 now
Changing version to '9' as part of upcoming Fedora 9 GA. More information and reason for this action is here: http://fedoraproject.org/wiki/BugZappers/HouseKeeping
*** This bug has been marked as a duplicate of 440055 ***