Being a Mac user at home I've been keeping a close eye on the OSX port of this. The developer is doing a great job, and has it running very nicely on a Mac Mini.
From what he's been saying (through his blog), I'm not sure which dual core Mac Mini he's got, but he played a1080p MKV of Rumor Has It, average bitrate of 11.5Mbps, spikes to 15-20, with only 230 dropped frames through the entire movie.
His CPU only hit 130-140% as well, so with some more tweak a Mac Mini will be capable of pretty much all 1080p content. IMO that will make the perfect OSXBMC device.
Now for someone to add a front end for Myth or HD Homerun's and I can finally do away with the last remnant of Windows in my house! (being a Vista MC).