Feel really quite torn by this. On one hand, being someone of a fairly geographic and map-oriented persuasion, being able to geographically track my friends and my life is immensely appealing. On the other though, at what point do you stop experiencing the world fully unless it's foursquared, facebooked or tweeted - nevermind the privacy issues.
It's the consumption of tourist spaces by photographs taken to the next level. The experiential nature of being somewhere has to be validated with evidence which, as it moves towards the realtime web, means that that validation inevitably dilutes the actual experience of being there.
As to foursquare, no iPhone and working from home kind of puts the mockers on that, but I do kind of wonder for how long.