As said before, an IP address is not personally identifiable information, my IP address is 82.21.53.245 - go nuts :P
Sure, someone could order an ISP to release more information based on that IP, but your IP address is logged every time, you load any page, from any site.
So, if you're posting things you shouldn't be on a forum, all it takes is for someone to register, stick an image in their signature that's hosted on their site - post in a topic, and bam - they have the IP address of everyone who's viewed that post.
Let's say they wanted to get the IP of a specific user: they send you a PM with an image in it, you view the image, your IP turns up in their server logs for that image.