I found a pretty cool Winamp Plug-in on oddsock.org today called "doSomething". The you can set the plug-in up to do a number of actions when Winamp changes songs—one of which is to perform a GET operation on a URL. Unfortunately, there appears to be a bug in the event handler that handles the actions when Winamp is either closed or a playing song has stopped. So, I had to create a hack to indicated when I'm offline by checking the length of the current song and when the song started. If no updates have occurred within that time, then I set the status to "offline"
Download the plug-in here: