Raspberry Pi has just announced the release of a fork a linux kernel 3.1.9. The source code with patches is available at https://github.com/raspberrypi/linux. If you just want to download the patch, I’ve created one: r-pi_linux_3.1.9.patch.gz
The code related to Broadcom BCM2835 processor is referred as bcm2708 in the kernel and I can see committed related to the watchdog timer, the framebuffer, the VCHIQ driver (the driver sending messages to the GPU) and general commits for bcm2708.
For those interested in the messages used to communicate between userspace and the GPU, the VCHIQ driver code is located at drivers/misc/vc04_services in the tree. I haven’t studied it yet, but as the platform should support graphics standards such as OpenGL and OpenMAX IL, most people should not really care about this interface as I suppose the OpenGL library (Mesa?) uses this driver.