In plain English, to qualify for a prize your hack will need to:
Use the foursquare API (duhhhh!)
Be started and completed during the hackathon weekend. Submit your hack to our voting site by the end of Sunday in your local time zone. (That's right, Rules Reader, you've got an extra day to nurse your hangover, fix some bugs, and polish things up.)
All common, public SDKs and services are fine to use during the hackathon. If you have an uncommon/personal open source app or framework you might want to use, e-mail us details and we'll list it in our Code Bin so it's definitely fair game!