Tipped off by a CNET blog post, I see that the latest version of Apple's Boot Camp solution, which lets you dual boot a Mac between Mac OS X and Windows, now only works with Windows 7. Previous versions also supported Windows XP and Vista.
According to an Apple support document, Boot Camp 4.0 works with Mac OS X 10.6.8 and 10.7 (Lion), and it comes with Lion. But on the Windows side, you can now only use an "authentic, single, full-installation, 32-bit or 64-bit Microsoft Windows 7 Home Premium, Professional, or Ultimate."
As with previous versions of Boot Camp, the new version comes with the drivers needed to get Windows fully working on your Mac, and newer drivers are periodically made available for downloading as part of the setup process.
I guess the bigger question here is, what, if any, improvements does Boot Camp 4.0 offer over previous versions? Does it perform better, offer better Windows drivers? Apple's list of 250 new features for Lion doesn't even mention Boot Camp, so it's hard to say.