Actually, no, that's what would be called a "cheat". Microsoft's policy is very clear about this kind of thing, and a boot camp install plus a Parallels install requires two licenses of Windows XP. Windows Vista Enterprise and Ultimate will allow this kind of install with a single license though.
Obviously, it is technically possible to do what you're saying, and the folks on the other end of the activation line might give you a pass (though that may be because they either 1) don't understand what you're actually doing or 2) don't understand the license issues). But let us not unintentionally deceive people here---the practice is not within the bounds of the XP license agreement.
More info here:
http://blogs.msdn.com/mikekol/archive/2006/07/12/662511.aspx
Check the comments for the XP clarification.
Last edited: Sep 28, 2006