Ext4, a incremental revision of ext3 is built from the same codebase and compatible to it while providing better performance and scalability but there are limitations to what can be done in a compatible manner. Btrfs is now a multi-vendor effort from Red Hat, HP, IBM and Oracle allowing a common pool to save costs of development and will provide a number of additional features which requires a fundamental redesign.
While Ext4 has long been merged into the Linux kernel as a development filesystem, it is getting closer to being marked as stable and beginning to see adoption from Linux distributions. Fedora 9 already includes ext4 as a technology preview and Fedora 10 will make it simpler for end users to adopt it as well although ext3 remains the default for Fedora and many major distributions.
Andrew Morton, a core Linux developer has indicated that he would like to get btrfs merged as early as 2.6.29 though it will be marked as a development or experimental feature for a while. This will allow btrfs to progress further as part of the Linux kernel development and will likely be the default for major distributions sometime in the future.