Linux Magazine has put up a video of Ted Tso, one of the primary developers of Ext4 and current CTO of Linux Foundation, on the current state of Ext4, the current plan of Fedora to have it as the default filesystem for Fedora 11, how it compares to Btrfs, and other insights.
Ted says that Ext4 is close to production-ready being built on the mature and stable Ext3 codebase and being an evolutionary step that brings in additional performance and scalability. Btrfs is a completely new filesytem developed from scratch with a number of new features like filesystem level snapshots that Ext4 could not accommodate. Ext4 is well on its way to becoming the next standard filesystem for Linux while allowing Btrfs to innovate more and mature over time.
Can’t wait for this FS, it will kick ass.