It's probably not authorized, but illegal would probably be pushing it a bit far. Some companies are already doing this. As long as you legally purchased the software, you can probably get away with installing it wherever you want. This is an interesting question though; I'm going to look into this further.
Related excerpt from the Mac OS X EULA:
"... you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at a time. You agree not to install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so ..."
So, I guess the answer to your question is, indeed, no - it's not legal. However, I doubt that if you do this in your own basement, anyone is going to care about it. You are, after all, paying for the software.