ATI is hardly behind at the moment. The 4870 (desktop) matches the GTX 260. The 4870X2 should easily match the GTX 280. The 4 series is so good that Nvidia actually lowered their prices from $650 for the GTX 280 to $500 and $400 for the GTX 260 to $300. Nvidia had no idea they would be so good.
I would hope for an Nvidia GPU in the next revision, as the 3650 is the 2600 with DirectX 10.1. Now you might say "OSX doesn't use DirectX!", and you're absolutely right. There would be no added benefit. The 9600GT is what we're going to get.
It's funny that some of you guys are convinced that it's a horrible card when in fact it's a mid range card. Before I built my current gaming computer along with my G4, I was using a GeForce 2MX, and a 5200FX in an iMac. THOSE are garbage cards, even for their time.
Compared to the 8800GTS in my computer, the 8600GT isn't much. But it's miles ahead of where you think it is.
As someone said above me, if the X1600 gets 35FPS with medium settings, that's great. 30FPS is considered easily playable, anything more is just icing on the cake.
Actually the GTX 280 is only a tad bit better than the 4870. The only game where the GTX 280 will be noticeably better is Crysis...which is still like 40fps versus 35fps, and $650 versus $299 respectively. In other games you're getting 60 and above; you won't know the difference. You're dumb if you grab the GTX280...and I guess most of America ain't dumb, that's why Nvidia is losing sales on contrary to popular belief. So no, ATi is not behind at all. It's just that they're smart enough to use a slightly different technology to control cost and not get their behinds bitten by Rambus in the process.