When it comes to the Radeon DRM driver in Linux 2.6.39, the main addition this time around is the Cayman GPU support. AMD published the patches earlier this month for Radeon HD 6900 series open-source support in the DRM/KMS driver, but there is currently no 2D/3D/video acceleration support with this code. For the R600 series, there is now tiling support that's enabled (it's already been this way for R300~R500 ASICs) along with predication support.
The Nouveau DRM driver for reverse-engineered open-source NVIDIA GeForce/Quadro support has KMS page-flipping now enabled by default (big performance win!) plus Z-compression. There aren't any other major changes and the GeForce 400/500 "Fermi" support in Nouveau still requires non-free firmware at this point.
Intel's DRM driver has a big i855 fix, output setup re-factoring, and miscellaneous other fixes. There is still no Intel G45 VA-API support. The older Intel i830 DRM driver has also been dropped completely in the Linux 2.6.39 kernel.
Some of the shared core changes to the Direct Rendering Manager in the Linux 2.6.39 kernel is removing the Big Kernel Lock (BKL) need, support for USB DRM drivers (to allow USB graphics devices such as those from DisplayLink), and a generic DRM capabilities ioctl. The TTM (Translation Table Maps) kernel memory manager for the Radeon and Nouveau drivers now has Xen Dom0 support. The TTM support in Xen Dom0 was worked on by Oracle back in January.
David's 2.6.39 DRM pull request can be found on the dri-devel mailing list.