Assuming the person being tracked is not being sneaky and using a proxy, you can tell who their ISP is at the very least. And if you know their ISP, and you know their IP, you can, assuming the poster breaks a law or whatever, contact the ISP and tell them that "whoever was using this IP addy at this time did X". Then, if the ISP decides to cooperate, the poster might actually get spanked.
Sometimes you can tell more than just who the ISP is. I logged onto Lady Jing's Web site and she was able to track me to within about 20 miles of my RL location, which is pretty damn good.