The latest Steam hardware survey shows almost a quarter of Steam's user base has already adopted Windows 7:
One of the biggest questions surrounding the Windows 7 launch would be whether gamers would adopt the new operating system, or stick with the tried and true Windows XP. The latest Steam hardware survey for December shows that almost a quarter of Steam's user base has already adopted Microsoft's latest OS, and has been abandoning the 32-bit Windows XP for 64-bit versions of Windows 7.
The success of DirectX 11 has been tied into the Windows 7 OS, but DX11 can also be installed on computers running Windows Vista. Over 50% of Steam users are capable of supporting DX11 on the software side through Windows 7 or Vista, but unfortunately Steam isn't keeping track of DX11 video cards yet.