When Windows Vista shipped, it shipped alongside DirectX 10. DirectX 10 would only run with Vista. It wouldn't run with the faster, more stable, more popular Windows XP. This made those running games on XP unhappy. Well, fast forward a few years, and the next release of a Windows OS - Windows 7 - won't be repeating the same mistake. Microsoft's Product Marketing Manager Windows, Ben Basaric, has gone on record saying that while Windows 7 will be shipping with DX11 support (which will add features like allowing a graphics processor to process more than just graphics), it will also run on Windows Vista. So no need to go upgrading. Well...unless you're one of millions still using XP...

DirectX 11 confirmed for Windows 7 - Microsoft revised statement [PCGH, via Voodoo Extreme]