I was wondering why they were being selective on the people who could come to this. I really wanted to come and learn more about their API but it seems as though they were picking people who already built on it. Isn't the point of these events to get new people into the ecosystem?
Send a message to my github account when we do the next foursquare hackathon and I'll do my best to get you in (https://github.com/tjulien). We maxed out the number of attendees based on how many people the space could hold. There was no criteria of having used our api before.
As I understand it, there was some vague talk about being selective, in case they maxed out the capacity of the space, but in the end I believe they sent out tickets to everyone who signed up, and nobody was turned away.
To answer the question, yes people really do code and come up with solid apps at these events. Check out the music hackday apps that were built last weekend at the same space: http://wiki.musichackday.org/index.php?title=NYC_2011_Hacks