One easy way is to give away prizes to the top 3 apps built at the hackathon. The reason why people actually coded at the iOS hackathon were the prizes (I attended). Obviously submissions built prior to the hackathon would be disqualified so this prevents people from building it, then attending.

There's still lots of socialization, but I believe hackathons should be centered around code, otherwise you might as well just call it a foursquare mixer. There were lots of really cool apps built at the iOS hackathon -- I'm really glad it was so hacker-centric, however the prizes were significant. Paypal gave away 3 iPads to winners that night.

Also, if the goal is to get people learning about the foursquare API -- the only way people will learn is by practicing. I learned a lot from others at the iOS hackathon -- even stuff I thought I already knew well. I think foursquare would get a lot more mileage if they just added a few small prizes to fuel competition.

Some of my friends are marathon runners -- they say that if the city sponsoring the running event offers no prizes, almost no professional runners will show up, but if the city offered a $100 small prize, there will be a significant amount of pro athletes attending. Does foursquare want pro hackers attending, or do they just want scavengers who are attending for the free food?