Hey, I heard about this on Radio 1 news today and gave it a try. It's pretty cool.
Basically you install a little java app on your phone (which works with the N95 internal GPS) and that uploads your position to the website map whenever it's running. The app lets you take photos, videos, type notes etc and they appear on the website map according to your position when you created them. You can also message other users and recieve alerts when your contacts are close by.
If you go on the website you can see where everyone is and also look at the content thats been tagged in certain places. Eg/ There are photos of the London eye at its location on the map.
It's only in beta now but looks like a cool idea. It even tells you if someone is 'live' (running the app) at the moment and if you want, you can zoom in and watch them travelling along! Obviously personal data is protected and you can choose to hide your position (from the phone app) if you want to tag things but not appear on the map.