So I fergot about this for a long while, last I checked in it was buggy as hell and barely starting let alone working well. Well I moved my wife's system into a room next to the home theater room and a set of cables came in on Friday from the fellas at Blue jeans, one being a 50 foot HDMI, the other a 50 foot toslink. I pluged them in, ran them through the ceiling and across the room to the pre/pro and started messing with highdef playback using VLC< perian on Quicktime, etc and they all sucked, at some point they'd lag or chop up or just drop playback. This is a 2.4ghz 4gb Imac, using a 4 terabytes of firewire 800 from lacie for storage - 24" btw. So anyway, I visited http://dn-0.com/xbmc-trac/ and it's very much working now. I'm very close to chucking my old 1.3 Xbox that's done nothing but run XBMC for the last several years. 1080p playback is flawless accross the board. External display driving on my 50" Kuro is perfect, DTS, DD passthrough works great format support as well, forwards backwards it's just like XBMC only i'm using the little mac remote atm.
If you're looking at a near perfect media center, XMBC on the mac is it. I guess the 1.8ghz mac mini's with 2gb ram are playing 1080p content just fine, as a fyi.