The blows away the if you compare tests on PC benchmarking sites; in fact, it can also mostly keep pace with the Radeon 4870 X2, a variant of the ATI card that sports two Radeon 4870 chips working in parallel.
Why wait until June? My bet is that Nvidia is holding off until Snow Leopard, the next major update to Mac OS X, drops. (Apple hasn't announced a release date for Snow Leopard, but it has scheduled its for June 8). Maybe Nvidia couldn't get the drivers to work the card into an interim Mac OS X update between now and then, or maybe the performance of the card will lend itself more to , the parallel processing technology in Mac OS X 10.6 that will enable "regular" applications to tap into the processing capability of a GPU.
It's hard to say how well the GeForce GTX 285 will work in a Mac, partly because the PC benchmark sites benchmark games that haven't come to the Mac. Different card makers overclock chips and use customized Windows drivers, which also effect performance.
What I can tell you is that the GeForce GTX 285 is one step away from the top of Nvidia's GeForce line (the GTX 295 is a two-chip card, like ATI's Radeon 4870 X2). This will also mark the debut of the GT 200 series chip architecture on the Mac. The Mac Pro (and iMac) both come with GT 100 series cards, which operate at a fraction of the speed of the GTX 285. The texture fill rate of the GT 120, the card that comes with the Mac Pro, is 8.8 billion textures per second; the texture fill rate of the GTX 285 is 51.8 billion per second.
How much will the card cost? Again, another unknown--neither Nvidia nor EVGA were willing to admit to the card when I spoke with them, though it looks like it's a sure thing--but PC cards currently run between US$340 and $400. So start saving your shekels.
Either way, things are looking up for Mac Pro owners--users of Apple's heaviest iron, who have been limited in their choices for graphics cards for far too long.