All the recent chat seems to be about Gowalla. It's understandable given their recent funding success. I'm just on Foursquare for now (heck; it only just launched up here in Edinburgh) and that's enough for me.
I noticed that some time today the mobile web version of Foursquare - I'm not talking about pretty apps for you iPhone users - got a minor upgrade with an "Send to Facebook" drop down box. This is the screen I tend to see most often as I use my Windows 6.1 X1 and the mobile Iris browser to check in remotely.