Boot Camp in Lion requires Windows 7

Apple's latest version of OS X 10.7 Lion ships with Boot Camp 4 which, according to an Apple support document, only runs Windows 7. Previous versions of Windows like Vista and XP are no longer supported. There's no reason for this change, but Apple, like Microsoft, is likely increasing its focus on the newer OS and lessening it on legacy versions.
Mac users that must run an older version of Windows will either have to keep Boot Camp 3 if they wish to upgrade to Lion or use a third-party virtualization solution from VMWare or Parallels.