Upstream has dropped the development tag from Ext4 in the latest Linux kernel release which is a indication that they believe it is good for more wide spread usage.
Red Hat has kernel filesystem developers working on Ext 3/4, XFS and Btrfs and Fedora has supported Ext4 as a experimental option right from the Fedora 9 release. Red Hat has included Ext4 as a technology preview in the recently released RHEL 5.3 as well.