You can run XMonad on a Mac, but it will only tile X11 apps, not standard OS X apps, which unfortunately makes it fairly useless for me.

And of course, working in a VM with it is always an option (and I have a VM setup with it for when I want to drop back into linux).

Does that include Chrome? Because I've definitely seen them use Chrome, and it looked Mac OS-y. Of course, the people I'm thinking of probably have a different outlook on OS X: they like it, but not the UI. (That's a paraphrase from one of them:)).

I've fiddled with xmonad on the mac, and as someone else said it only tiles X11 apps. This is not totally useless as I primarily like xmonad for controlling terminal windows, so I set it up to manage urxvt instances. I used xmonad.layout.gaps to make it avoid my dock area (I like my dock on the left, and I like to have the dock even when xmonad is around) and it seems to work well but the configuration breaks in subtle ways (doesn't start up sometimes, etc). I haven't put any more time into figuring it out. I might just go the VM route to get my xmonad fix. If Lion's x11.app had true Lion fullscreen support it might work better, but it doesn't yet.