The 2.6.38-rc7 release announcement in full says: "There really isn't a lot to report here. Driver updates (random one-liners and some sound soc codec and smaller dri updates) and a few filesystem updates (in particular btrfs fiemap and ENOSPC handling), but most of it really is pretty tiny. Regressions fixed, hopefully none introduced.."
The final Linux 2.6.38 kernel should be released in the near future with its Radeon HD 6000 series open-source support, AMD Fusion Linux support, ATI page-flipping, Nouveau Fermi acceleration, and many other open-source graphics improvements. This kernel to be used by Ubuntu 11.04 also has EXT4/Btrfs changes, pulls in a kernel patch that does wonders, and offers other kernel improvements.
There are already a number of Linux 2.6.38 kernel benchmarks on OpenBenchmarking.org.