Google Latitude  is a great service. You can install it on most mobile phone platforms (S60, PocketPC, Blackberry, iPhone/Android are yet to come) and can share location information with your friends on the mobile phone (or also on the PC) at differen levels of detail, no location sharing, city level, or precise location.
Having it installed for a few days I still experience it as a gadget, it's fun to check whether my and my colleague's actual location really matches with google's prediction and quite often it doesn't - anyway, that will change quickly, I'm sure.
The set-up of the application is rather simple, especially because of the fact that google already knows so much about me, I had been sharing with me them voluntaritly for quite a while: of course I use gmail, so I already have the account. The pic for my profile I just can take from picasa, I can link this blog to my profile, there might the social network, my experiences/videos on youtube...and the location, suddenly it fits altogether! I wonder how I react, when the US immigration officer tells me in ten years "you can keep you passport in your pocket, Florian, we saw you coming and have already identified you through your google acount!"...a new feature perhaps called google passport?
Currently, google latitude really reduces the where-are-you/when-do-you-arrive calls. While I'm cycling home, my wife can see which route I take, whether I go straight home or take some lovely detour through the forrest - and that's with my consent that she knows about it. I wonder though whether the police will bill me in ten years for all the one-ways I cycled through in the wrong direction. Currently, Google denies to store any tracks just the current position. But that could change and Google could report my sins...
I also wonder whether new services, let's call them 'correctizers', might appear that allow me to 'correct' my current location or also to 'repair' my history trace.
I don't think we will live in a big-brother world soon, I guess it will rather be the 'small brothers' we share information with either voluntarily or also by social pressure,  envisions this rather nicely.
What was also intriguing me is that most of my colleagues - researchers in ubicomp/internet of things areas, dealing with location-based services etc. - just declined my inquiry to share their location with me. Obviously, privacy becomes important when you really can feel it...
 David Brin: The Transparent Society: Will Technology Force Us to Choose Between Privacy and Freedom?, Perseus Books Group, 1999.