Windows 7 Professional and Ultimate is stated to having an XP mode where applications that only run in XP can be installed in a virtualized XP environment and linked to the Windows 7 desktop. Microsoft is including this feature to hopefully ease businesses into upgrading with the assurance that the critical business apps will still run. This is one of the big reason Vista still hasn’t been adopted by businesses. This feature should be in the upcoming April 30th Release Candidate.
Source: The Register