One thing for which it might be useful in my personal context: real-time traffic updates. It's easy enough to find a Twitter client for a smartphone (I did give TinyTwitter a try) and "tweet" what you're running into during a commute, so people who are following you can avoid it. The problem would be knowing who to follow to get the updates you want. There's also the possibility of giving a stalker a literal roadmap of your daily commute and how to find you.