The site would be able to determine that. In a chat room its easy to find out or p2p. But if they are just postings on a blog, it would have to be the sites owners to find out. Even if you did, IP’s can be dynamic or grouped into a common range. In addition, many peope surf through anonymizers and proxies. I agree, just ignoring them is the best bet.
Is it easy to trace back on AIM?