AS stated on the mobileMe web page, if youn remote wipe it, you won't be able to track it anymore.
Remote lock is a better option.
Though in reality all they have to do is remove the sim (or turn it off) and puff it is gone.
Then if restored they can just set it up as a new phone.
By the way. If your phone has been stolen, is it showing in mobileme, and if so, why havn't you told the police where it is?