My post above contained a tracking beacon which was logging IPs & useragents; the link in my post went to a free hosting provider which I set up using a manner of techniques to log information about visitors (using JavaScript, PHP, and an embedded flash player which was requesting a video from my server), and then after a few seconds forwarded you to a legit blog post.
Fascinating! How can you possibly embed such code in a forum post? Surely this indicates a serious bug in SMF, the forum software?
One of the IP addresses you mention is mine, and I'm not joe23. Thanks for doing the xx.xx'ing - I'd hate to have a bunch of you guys suddenly trying to hack my box!
As theymos said, just an image: a simple 1px transparent gif hosted on a server which logs IPs of those who requested it.