Well.. you could but you would need the IP address also.

You just need to check if the request is coming from a T-Mobile IP, or any other GSM carrier in the USA (http://en.wikipedia.org/wiki/List_of_United_States_wireless_...).

This wouldn't work outside the USA (unless you want to research the carriers) and most people would be connected to wifi.

So, this wouldn't really work. It would be much easier with a BlackBerry as they transmit the vendorID (a type of carrierID) in the UA. You could easily check this with the IP.