Core i7 iMac, 8gb RAM, Windows 7 64bit here. Game runs absolutely sweetly, and I can *almost* tolerate playing it on max everything bar AA and HBAO (the latter is a frame rate slaughterer) at 2560x1440.
The one thing that frustrates me is that 1920x1080 is a perfectly acceptable resolution, albeit not quite as sexy as native, but I can't for the life of me get the game to run full screen without black borders at this resolution- I managed to do it by hooking up a 1080p, 24" monitor in mirrored mode... but that's a sub-optimal solution for sure.
Installing the latest ATI Catalyst drivers on the iMac don't seem to yield the Catalyst control panel, where I assume you'd usually turn on stretch-to-fit to the LCD panel.
On the i7 iMac it's really a trade-off between max everything and 1080p, or just a little detail reduction and 1440p. Obviously the frag-happy purists will favour low everything with a high resolution and high FSAA/AF but it's refreshing to have a game look so beautiful after stepping away from PC gaming for so many years.
A Mac port is almost a certainty, but I don't hold out any hope of it being truly native, or offering the level of performance I get from the Windows version. Let's hope to hell that Steam block Cider wrapped crap from their service. I'd be wanting to play the Mac port in ultra low detail on my 13" MacBook Pro, for those quick fixes, so performance matters!