I am not a MobileMe subscriber because I found I never used it when I did subscribe back when it launched. If you have an iPhone there is a new service called Find My iPhone that provides you with ways to track a lost iPhone. I just read this real-life story of how a lost iPhone was recovered, but do not recommend people take this much direct action to recover a lost iPhone. Remember, it is just a phone and can be replaced and even wiped clean too.
The post author and his buddies used a laptop and Sprint wireless modem to track down his lost iPhone using the Find My iPhone service and were luckily able to recover the iPhone safely. The thing is, do you really want to go face-to-face with the thief who took your iPhone? Even though I am a pretty big dude, I don't really value my phones that much and probably would not have gone as far as these guys. Also, do NOT call 911 if you lost your dang phone. The police have more important problems to worry about than recovering your lost or stolen phone.
Would you go this far to get back your iPhone? If people continue to use the service in this manner, I wonder if Apple will shut it down because it is too dangerous.