Ending the debate as to whether XFS will ever be merged [earlier story], it can now be found in Linus' 2.5 BK tree, and will be part of the upcoming 2.5.36 kernel. You can see the XFS-related BK changeset details here.
XFS is a GPL'd high-performance journaling file system under development by SGI since 1994. Version 1.0 was released for Linux on May 1, 2001, with the current release being version 1.1. XFS joins several other journaling filesystems available in the mainline Linux kernel, including ext3, ReiserFS and JFS.