ext4 disaster

[Neal Becker]

After updating from F10 to F11 using ext4migrate of my root partition (not 
boot) I noticed that root was not ext4.  I booted off dvd, used tune2fs to 
turn on extents, ran fsck.  On reboot, the kernel would not mount root, 
saying it was using unsupported features!!  I tried fixing with tune2fs, but 
tune2fs refuses to turn off extents.

I did a clean install of F11 from scratch, choosing default ext4.  Rebooted, 
and guess what?  Same thing!

Finally I did another clean install using ext3 and I'm back.

Big question: why did anaconda mount ext4 fine, but when I try to boot off 
my hard disk the (same kernel) won't?