I have to disagree with some above... the real answer is "it depends". Some IPs can actually be resolved to a physical address. It depends on how big the user is (usually). For example, big law firms, banks, etc. usually can be found (I like to use
www.samspade.org as a first try), although single home users very rarely can.
Also, some ISPs can be very helpful and some won't do shit. They all have the ability to track the user, even if they have a dynamic IP, if you give them both the IPs and the times they were on that IP. It's a question if they want to or not. Just yesterday I got a threat from som guy who I threw off of a board to hack my site and keep it down for months. I contacted his ISP with a copy of the threat, and they wrote back to me that they wouldn't do anything without a subpeona and that I should contact Law Enforcement to make a complaint. I wonder what thy would say if their CEO's home address and phone number showed up on my mesage board and I was equally "cooperative" about taking it down?