From Fedora Project Wiki
GRUB with Ext4 Support
Fedora 9 originally included experimental support for Ext4 and Fedora 11 included Ext4 by default however GRUB in that version did not support Ext4 and hence required a separate boot partition formatted as Ext3 or Ext2. Fedora 12 now includes a updated version of GRUB with Ext4 support. Anaconda, the Fedora installer will permit this as well.