AMD now also supports Windows 7 with new graphics drivers. NVIDIA announced Windows 7 beta drivers for GeForce cards 2 weeks ago. AMD offers Windows 7 driver support as part of the latest ATI Catalyst 9.3 unified graphics driver.

AMD says that the ATI Catalyst 9.3 represents the industry’s first unified driver installation package to incorporate Windows 7 support, including Windows Display Driver Model (WDDM) 1.1 compliance, bringing with it a host of benefits for developers currently testing and updating applications using the widely available Windows 7 beta release.
The combination of Windows 7 and ATI Radeon graphics permits developers to fully utilize the DirectX 10.1 API used by Microsoft to design the Windows 7 Aero desktop. Graphics with hardware support for the DirectX 10.1 API, currently available in ATI Radeon HD 3000 and ATI Radeon HD 4000 series products, can deliver the full Windows 7 Aero desktop PC experience, including support of new features such as Aero Peek and Aero Shake, both designed to improve navigation of the Windows 7 desktop.
More details on the AMD site.