Oh wait, I also did the stalker ads idea, or sort of. I believe that someone will come up with a successful solution for location-based mobile coupons. What we have seen with our project is that this idea clicks with customers, but it is hard to develop a scalable model to acquire local advertisers. Local business owners seems extremely reluctant to embrace the internet medium for some reason (even with some free limited service, free pilot programs, etc.).
It's interesting that customers like it. I started working on something like that, but the thought of being on the receiving end of constant advertising through my phone as I walked around really bugged me so I dropped the idea.
An explicitly opt-in version I could go along with, but the legwork to acquire advertisers was more than I wanted to do.
I hate to play devil's advocate, but that kind of crap is the reason Facebook apps exploded. You don't get viral growth without supporting friend-spam.
It's actually not required for signup. You would still receive coupons for free up to a face value of $5 (which is the vast majority in volume).
That was an incentive to foster virality, but your comments are very much appreciated.
I end again with a shameless plug: http://www.moqpon.com