Who? Someone on the forums? Someone you chat with? Someone in real life?
If you can get them to email you, their IP might be in the headers. I think Gmail blocks their IP, so if they use Gmail, that won't work.
Get them to go on IRC and you'd have it easy there.
I don't know if this still works, but if you're instant messaging them, you might be able to either set up a file transfer with them or a direct secure connection (does AIM still do that?). Once you're transferring a file, go to your command prompt and type 'netstat' to see all the IP addresses connected to you.
Set up a web page that logs IP addresses and tell him, "OMFG, look at this cool web site!"
If it's someone here on the forums, slip me fifty bucks and I'll tell you.