For my last semester, I (along with a couple of my fellow students) researched mobile communication as a base for a mobile calender for all kinds of cultural outgoings in this one city near me. Based on our research, we adviced this company on how to develop a cultural mobile calendar (on mobile phones, obviously) and on which functionalities it should have.
We discovered that the core functionality of this calendar should the the calender itself, but as one of the extra functionalities, there ought to be a way to see where your friends are so you can find them when you're going out, so you can meet up with them. We decided that integrating this with Google Maps was the best idea.
Funnily enough, not a month later, I read about Google's Latitude:
Looks way familiar. At least we know we did something right. I wish we'd got it developed now, though. Hee.