For everyone who is having trouble decoding HD content, you may consider installing mplayer 1.0~rc2.
I've uploaded it to hardy as of a week and a half ago. It was approved for a backport to gutsy as of yesterday. It should show up on gutsy-backports within the next few days as soon as an archive administrator pokes it.
In the interim, i've got it on the PPA listed here for the interested: