I've seen multiple variations of similar requests. It seems that there's two separate problems that need to be addressed. There's the broadcasters problem where people want to broadcast information only to select groups of people. Facebook has taken a stab at this, Google+ has done it much better, and the concept is merely Public/Private for Twitter.

Then there's the subscriber problem where people want to read relevant information on a specific topic. This problem only seems to exist when connections only need to go in one direction, i.e. following. Here users want to be able to view posts on specific subjects by those with "celebrity" type persona's. Twitter solves this (IMO, poorly) with the use of hashtags and multiple accounts. The hashtags don't solve this problem completely because you can't subscribe to a specific user's hashtag. I don't personally care that @pydanny is drunk at a wedding with Audrey, but I'm super interested in his DjangoCon talk. I also don't care about Joe Schmoe learned the power of decorators in his #django views.

To me, tags don't seem like the right approach because of the reasons above. I personally like the concept of "Channels". Users can create any number of channels for whatever they want to post about. When you are adding a user to your circle, you can select which channels you'd like to subscribe to. Then when they post they can select which channel the post is relevant to. Now, some of you may say: Whats the difference between this and tagging? Well, the distinguishing factor is that you need to create channels prior to posting to them. It forces users to categorize their subjects consistently, and it allows subscribers specific choices on what they'd like to see.