Wasn't this actually an app from a social networking site in the US, its your contacts on that site and not your contacts in your phone and it was US only.
if you search for the WWDC08 video you should see the the name of the app/site in the video.
quick google and you will find its called loopt - Loopt - I fully expect there to be lots of location-aware social networks forming around the iPhone, but I expect Loopt to get a big boost for being featured in the keynote.