Please see:
http://technet.microsoft.com/en-us/library/cc731692%28WS.10%29.aspx[
^].
I never did it with IIS, but there are many forum Questions and Answers related to this feature (did you look for them?), and it looks like this is a pretty usual request; and all experts say it won't be a problem. So, I though it would be safe to say that this is not really a problem and refer you to this documentation. Same thing, even easier, goes with Apache.
Your problem might be having several different domains themselves, but this is just the problem of DNS and, usually, your Web hosting provider, and your hosting plan. Normally, to obtain a top-level domain, you have to register it in your name and pay some fee. I hope you know all that stuff, mentioned it just in case.
—SA