This is actually a part of MobileMe (as well as the remote erase).
With MobileMe, you can log into your account and if your phone is on, you can see right where it is (or, approximately)
One other thing you can do is send a text mesage, with the option of playing a sound for 2 minutes - whether the phone has been muted or not, it will play that sound as loud as possible.
I'm currently on the 60 day trial, but I have every intention on paying the $100 yearly fee for MobileMe - totally worth it, IMO.