According to the Foursquare blog:
We think a lot about the future of location, and, every time a new technology comes along, we think about how it affects what we're doing. Near-field communication (NFC) is one of those technologies; you tap your phone against a marker and it can check you in with foursquare. We've been testing it in our office for a little while and now we're expanding the test a little bit for Google I/O.
Google I/O is the big annual developer conference in California. This year, Google worked with us to put NFC check-in points all over the convention center. Starting tomorrow, just tap your NFC-enabled phone (most newer Android devices have it) against the poster, it'll check you in with foursquare, and you'll get a special Google I/O badge. For those without NFC, you can also scan the QR code on the poster itself.
NFC is, obviously, a long way from being available everywhere and in all phones, but we're excited by some of the potential. Google I/O attendees: have fun tapping in.
Foursquare revealed it was testing NFC internally in March this year.