A scene in The Matrix has Morpheus asking Neo in regards to the Matrix:  “Do you want to know WHAT IT IS?’ 

I have been seeing people post tweets about it for a few weeks now.  I saw this article by doing a Twitter search on “Foursquare What is it”

Foursquare Fun: Newest Social Media Site Is Complement to Twitter | theledger.com | The Ledger | Lakeland, FL

Looks interesting.  I’m in.  Or about to be.

I would bet that smartmobs.com has an article on it,  since it seems that foursquare is geared to/example of smartmobbing location apps