I was able to enable it in Jaunty by adding the avenard repos.
http://www.avenard.org/files/ubuntu-repos/
And installing vdpau, the nvidia drivers, and an updated mplayer. The video output in mplayer must be set to vdpau. This only works for files though, if you wish to play bluray discs you'll need to try something else. I didn't try this on a computer with the system 76 drivers though so it may have other issues. Another option is to simply compile the software you need and install the version of nvidia drivers you want manually. You may need to make a special launcher with the correct mplayer commands to activate vdpau for the file type you use.
EDIT: The mplayer version in Karmic does come with the vdpau output option in preferences. There's also a driver in the repository for vdpau support. It's called nvidia-185-libvdpau.