The Apple Store is now selling the cards after a quick 2 day wait.
But they are $350!!!!!
I guess I should not be surprised by Apple overcharging, but you can get this card for PC for $150.

Ships: 5-7 weeks
Overview
For incredible graphics processing power, the ATI Radeon HD 4870 with 512MB of GDDR5 memory is the ultimate choice for visual creative, scientific, and technical applications.
ATI Radeon HD 4870 includes two video ports: one Mini DisplayPort and one dual-link DVI port. This allows you to connect both the 24-inch Apple LED Cinema Display or another Mini DisplayPort-based display, and a DVI-based display such as the 30-inch Apple Cinema HD Display. Display adapters are available to connect VGA displays.
Compatibility:
Requires Mac Pro (Early 2009 with 1066MHz DDR3 memory) or Mac Pro (Early 2008 with 800MHz DDR2 FB-DIMM memory) with PCI Express 2.0 slot
Mac OS X v10.5.6 or later
But they are $350!!!!!
I guess I should not be surprised by Apple overcharging, but you can get this card for PC for $150.
Ships: 5-7 weeks
Overview
For incredible graphics processing power, the ATI Radeon HD 4870 with 512MB of GDDR5 memory is the ultimate choice for visual creative, scientific, and technical applications.
ATI Radeon HD 4870 includes two video ports: one Mini DisplayPort and one dual-link DVI port. This allows you to connect both the 24-inch Apple LED Cinema Display or another Mini DisplayPort-based display, and a DVI-based display such as the 30-inch Apple Cinema HD Display. Display adapters are available to connect VGA displays.
Compatibility:
Requires Mac Pro (Early 2009 with 1066MHz DDR3 memory) or Mac Pro (Early 2008 with 800MHz DDR2 FB-DIMM memory) with PCI Express 2.0 slot
Mac OS X v10.5.6 or later