This is a really good news for Linux-based HTPC owners. Mplayer recently added supports of HD-DVD/Blu-Ray codecs. The latest code in Subversion repository can do it with that DRM decryption patch. The full guide how to play and rip HD-DVD and Blu-ray disks under Linux can be found here.
I was asked a few times about possibility to play Blu-ray disk under LinuxMCE. So, with solution below it could be possible to do that.