I wanted to test VA API with my Lenovo T400s (GMA4500HD aka G45). You first need to install libva and then a libva-enabled mplayer. The only problem I have encountered is that the Debian package in the libva I downloaded disabled the i965 driver. I have reenabled it, it seems working. After installing, I found a small problem, reported on the Phoronix and the hkepc forums both: the skin of the people become blue. This can also be fixed easily.
apt-get build-dep mplayer
tar xzf libva-latest.tar.gz
echo debian/tmp/usr/lib/va/drivers/i965_drv_video.so >> debian/libva-dev.install
sed -i s/disable-i965-driver/enable-i965-driver/ debian/rules
dpkg -i libva*deb
tar xjf mplayer-vaapi-latest.tar.bz2
/usr/local/src/mplayer-vaapi-*/mplayer-vaapi/mplayer -vf rgb2bgr=swap,scale -vo vaapi -va vaapi whatever_hd.mkv