Well, it's illegal for one reason. A retail license is usually for one computer only. Your MSDN version might be different though, you may be able to generate a few keys. In a TechNet subscription you can get multiple keys (10) for each version.
And from what I've heard you can successfully activate each key on at least 2 or 3 different machines. I would assume this is true with ordinary licenses as well, but regardless, it's still a violation of the EULA and possibly illegal (not entirely sure on that part)
