Well, if I got the app, and some idiot started posting racist rhetoric or links to pics of their penis, i'd rather get fewer than an unlimited amount.

Additionally, people should be able to block posters, and the weight of those blocks can ban a user if there are enough and the velocity is high.

In that vein, it would seem that an IRC approach with "channels" might be better, so that you can have a private "channel" with your friends nearby, or move into the "starbucks" channel to get deals and updates from your local Starbucks.

Perhaps allow people to create "private" channels, by invite only, if they buy the paid version of the app..