marc andreessen did a talk a few months ago at Stanford (get it at ecorner.stanford.edu) where he talks about how the mobile app market looks a lot like the early days of boxed software - a million half-decent attempts but nothing really amazing.
and then came microsoft.
the world of mobile apps seems at present to be an arms race and most of that $20m is likely going to be spent on the guns and bombs of the tech world: great engineers.
The foursquare guys really do believe in the idea. If I had what I thought to be a great idea, and someone canned it and told me to do some other work, I would quit and start over too.
The real question when they raise at that high of a valuation is whether someone pay several hundred million for them.
http://www.sfgate.com/cgi-bin/article.cgi?f=/g/a/2010/04/06/...