Speaking with Gamasutra, Stardock CEO Brad Wardell states "it would be good if everybody switched to Windows 7 as quickly as possible."
He specifically points to WARP as a major contributor to his support of the forthcoming operating system. The Windows Advanced Rasterization Platform allows a PC's CPU to perform DirectX acceleration if said PC's video card doesn't support certain rendering features.
"One of the things that comes up often is, 'How has Stardock made so much money on these niche games?' Well, because our games run on millions of boxes."
Wardell's philosophy of accessibility seems to be one prevalent among successful PC developers, including Valve and Blizzard Entertainment. For example, Valve's support of legacy hardware allows users to run Team Fortress 2 in DirectX 8 mode.
Personally, I'm a big fan of Windows 7. To date I have tested Fallout 3, Half-Life 2: Episode 2, Prototype, Team Fortress 2, The Sims 3, Warhammer 40,000: Dawn of War II, and more on Build 7100 of Microsoft's new operating system, with all games performing similarly to, if not better than their counterparts on Windows XP x64 edition. Those interested in testing the release candidate can find instructions here.