Since the release of Linux 3.7 in early December there have been many changes presented for Linux 3.8, which are covered in the Phoronix 3.8 feature overview article. Among the highlights are the introduction of the new F2FS file-syste, EXT4 inline data support, true CPU hot-plug support, Intel i386 support removal, new processor enablement, Radeon driver performance improvements, Intel DRM work, and a whole lot more.
For those interested in benchmarks of Linux 3.8, among the in-house Phoronix tests to date include F2FS file-system benchmarks, EXT4 and Btrfs on Linux 3.8, Intel Ivy Bridge HD 4000 benchmarks, a Nouveau 5-way GPU comparison, NVIDIA GeForce 600 Linux performance, faster AMD Radeon graphics, and other 3.8 benchmarks.
Linus announced the Linux 3.8 release with this mailing list message. There's still a Btrfs fix that wasn't merged for Linux 3.8 but it looks like that will be staved off until a stable point release.
Next up now is the Linux 3.9 kernel with its merge window opening this week. There's already a lot of fun merges planned for this next kernel development cycle.