From AppleInsider:
Microsoft’s new software will eventually be bundled on every new PC sold, outside of Apple’s, so there’s no need to worry about leaking features or showing off the flaws of an unfinished product. Microsoft doesn’t have to sell Windows 7, it only has to worry about market rejection. Due to the volumes of PCs it will eventually be installed on, it’s bound to be successful even if it is a marginal product.
You should read the full note; the approach taken by Apple to 64 bit processing in the upcoming operating system, Snow Leopard, is clearly explained.
Via Windows 7 vs. Mac OS X Snow Leopard: Apple ups the ante (AppleInsider)