I really hope this is the right forum, if not you have to excuse me.
The license agreement found on: http://foursquare.com/legal/terms clearly states that:
If this is the case, how come there are tons of application & services out there today using the Foursquare API and content in different ways? Many of these application & services are being promoted by Foursquare themselves at their website /apps.
In addition, would you really create an API if nobody could use it unless it was for private use?