Auto Text Message????

Last night i was out at a local spot and I was talking to a guy about my marketing business and some plans that we were planning to offer to new clients. He then brought something up that struck my attention immediately. He began to tell me about someone else who was either using or developing a program that automatically text messages people as they pass a business.

For instance...say your driving and you pass a tanning salon, then all of a sudden you get a text message from the tanning salon with an advertisement for a free tan. Is this even possible without having the possibly clients cell phone number?????? Or is there something like it for people who are already existing clients (opted to receive text message promotions)?????

I think this is an amazing idea if there is some way to get it to work!!!! whats do you guys think????