Russell Buckley has written an insightful and honest paper on his experience conductung mobile promotions in two shopping malls in the UK. You can obtain a copy from him by following the instructions here.

It's interesting how much trouble he had to go through to emulate situational awareness: users, although they largely liked the service, had to specifically register and unregister when they entered the mall. Not surprisingly, not many did after the initial novelty wore off. One of his insights is that "passive user tracking" (i.e. technology takes care of figuring out where the user is, without requiring the user to do anything after they have given permission) is absolutely essential, which makes a lot of sense.

I'd like to go one step further, as follow-up to what I wrote earlier about "physical" vs. "logical" situations: location is an important criteria for whether or not a promotion is of interest to a user, but it's broader than that: e.g. if the user is part of a group of business people going out to lunch at the mall, chances are that they are far less interested in a special discount for, say, a CD at the record store than when the user walks along the same path at the same time on their day off.