Lookout Security has released an app called "Plan B", that you install AFTER you have lost your phone.
Using the new web version of the Android Market, you can now PUSH apps to your phone. This app claims to run automatically once it is PUSHED. It then sends you emails of your phones location, automatically turning GPS on.
You have already lost your phone, what else do you have to lose?
Probably wouldn't work so well if your phone was stolen, since it sends the emails to your Gmail account...but it could be worth a try...
You can find the app here: https://market.android.com/details?i...=search_result
I am in no way connected with Lookout, just saw this and thought someone might find it useful....
Edit: Forgot to mention...It's even a FREE app..