The only thing holding Foursquare back is GPS technology
To be honest, I think checking in is obnoxious and un-natural. That's not to say I don't do it, but it's disruptive to the social flow, and so getting it to be habit with a critical mass will be incredibly difficult. Some my friends don't know where their phone even is at any given point in time.
In a day not too far in the future, civilian GPS technology will be accurate enough to enable the coveted "auto-checkin." When that day comes, the gate is truly open, and we're off to the races. Everybody will try to catch up, but Foursquare already has the head-start.