Dan Frommer has a few good thoughts on this UI design trend:
A micro-trend in iOS apps? The new Facebook and Foursquare apps have buttons hovering over — and partially obscuring — the content below.
And it could quickly become tired/overdone quickly — you really wouldn’t want to overuse this. But I think we’ll see more of this technique.
I use Foursquare all the time to check into places, and I find the big "locate me" button to be really great. It's big enough to see and easy to tap with my thumb. To me, it's a perfect example of functional one-handed use of your phone.