Vista EULA Restricts Virtualization Use
As first noted in a HardMac forum post, it appears Microsoft is specifying in its license agreements that the ability to operate in a virtualized environment is prohibited in Home Basic and Home Premium editions, leaving users to have to purchase either the Business or the Ultimate versions of the software to legally run in a virtualized environment.
Mac users have found virtualization solutions such as Parallels Desktop a good way to switch between Mac OS X and Windows. This latest news from Microsoft may inhibit some use of the software as Business and Ultimate editions range from $300-400 verses Home Basic's $200 price point (prices).
Microsoft's EULA agreements can be found here.