Everything works, even H/W accelerated OpenGL, xv ... :-) However, I only got dual-head to work with x.org X server. Since I don't expect x.org to appear in Debian/unstable in the next couple of years I followed this excellent tutorial (Installing a non-intrusive X.org server on Debian).
Have a look at my x.org config and adjust it to your needs.
If you want to go with the Debian XFree86 4.3 server I got a config too.
glxgears brings 3196 frames in 5.0 seconds = 639.200 FPS in the default window and 459 frames in 5.0 seconds = 91.800 FPS in 1024x786.
X-Video Extension version 2.2 screen #0 Adaptor #0: "Intel(R) 830M/845G/852GM/855GM/865G/915G Video Overlay" number of ports: 1 port base: 60 ... maximum XvImage size: 1440 x 1080 ...
I do not use framebuffer anymore. I got a serious problem when the generic framebuffer driver was loaded before i915 (DRI Kernel module). Since I don't have any application using framebuffer I kicked it out of my Kernel. The Intel 810/815 Kernel module works though.