Twitter has a wonderful Location: field, and a lot of clients, like TwitterFon (for the iPhone), or twibble (for Symbian devices), tend to update the field automatically. They tend to update it with the phone’s built-in AGPS, so at worst, your accuracy in location, is about 100m or so.
Who cares when you’re in a public location? That can be a good thing for bumping into friends. But what about stalkers who now know where you live? Or an angry wife, that knows you weren’t working late, but chilling out with the guys at a nearby pub?
This is where Google Latitude kind of makes sense. Its opt-in. You only share the location with your friends. Twitter is just open (very rarely, do people’s profiles remain private).