Running a server without your ISP knowing..

[Omega]

I don't think they can really prevent you from running a server, just discourage it.  They usually use dynamic IP's to do so.  There can be multiple connections to/from your ip, without you being a server anyway.  I also don't think that they have anything against sharing your connection with your home network either, so there's no reason you'd have to spoof your address.  I don't think there's any way they can really prove you have a "server," just say that you were messing around with your settings in your OS, so that when you go to your ip, it brings up a web page, or ftp login, or whatever.