More specifically, it's an issue with ATI's current beta Windows 7 drivers not providing support for OpenGL:
Known Issues
This section provides information on known issues that may be experienced under
the Windows 7 operating system with the latest release of the Catalystâ„¢ Windows
7 Preview Driver Package. These include the following:
• Open GL is not supported under Windows 7 - 64 bit editions.
By "not supported" they mean "if it breaks, don't come running to us about it" - OpenGL does "work", as you discovered, it's just (A) buggy and (B) not hardware-accelerated, so it runs in sloooow software mode. It might run a bit faster if you turn the image quality down to Low on the options screen (Low image quality is specifically designed to speed up software rendering).
Presumably ATI will add full OpenGL support to a future version of their driver, before Windows 7 leaves beta.