I recently bought an Asrock ION 330 Pro HTPC which supports VDPAU, an NVIDIA API for Linux that offloads parts of video processing to the GPU. The upgrade from the old HTPC went smoothly with a mere dd if=/dev/sda of=/dev/sdb and a few system tweaks. However, it turned out that Mplayer in Debian Lenny that I was running on my new HTPC box did not support VDPAU. Checking the usual places, like backports and debian-multimedia did not help, either.
Building from source turned out to be exceptionally easy, though. NVIDIA provided a set of patches to Mplayer until VDPAU got integrated into the main tree somewhere in the beginning of 2010. The patches came with a shell script to check out a particular version from Mplayer SVN, apply patches and build. Mplayer source has a "debian" subdirectory with packaging information, so the only thing I had to do was to pick the latest patch set, install the development environment with apt-get build-dep mplayer, install the vdpau library and the headers with apt-get install libvdpau1 libvdpau-dev, run the shell script supplied by NVIDIA to check out the source and apply patches and… build the Debian package with debian/rules binary.
Here's the the resulting package for Lenny: mplayer_1.0svn_i386.deb
P.S. Add the following to ~/.mplayer/config to enable vdpau by default: