there was a thing on the web a couple of years ago called not melvins machine. showed somebody logging on to his stolen computer over the net and eventually getting the guy who took it and his computer back. don't say it can't be done.
The means to track after lost or stolen must be in place before the theft or loss occurs. Otherwise, you are down to random luck.
This topic comes up fairly often on Tom's and the answer is always the same. That being said, technically, it is possible to track down a system on the 'Net, but it VERY unlikely to be successful (wthout a tracking means in place). Your odds of winning the lottery are actually better.