I'm sorry if this is considered bad form, but I'm multi-posting this message to the various threads I've found asking for a Mac version of W(in|ow)AceUpdater, as it's directly relevant. I'm not trying to out-do WowAceUpdater when the Mac port of that arrives, but as this was only about 4 days' work at a couple of hours a day, I thought I'd let people play with it until such a time comes.
I decided that, over Christmas, in between festivities, I'd teach myself Objective-C and Cocoa and write something native for a Mac. The only thing that came to mind was a Mac port of WowAceUpdater, so I've written a native OSX version.
It's still very early days, but it works just fine for me, for what that's worth.