This is nothing new, but I thought I'd better at least have a link in this blog so I wouldn't keep forgetting about what exactly its name is!
Meetro, a location-based social software, and a great example of a buddy-tracking mobile app.
However, this would easily be superceded by anything similar launched by mobile operators, e.g. Vodafone. An even bigger threat is Microsoft - in a B2B sense, they've already got the context-awareness thing planned for the next rollout of Windows Mobile. All they'd need to do is figure in some sort of context-awareness for versions of MSN Messenger on Windows Mobile-run phones, and there's a good chance of widespread adoption by consumers. I really think Microsoft has one of the best shots at the mobile social software pie - if Vodafone doesn't beat them to it.