Sounds wrong, doesn't it? The circles sharing model is backwards-- why should I have to make guesses about what other people are interested about?
I've written about this on a previous App.net HN thread (http://news.ycombinator.com/item?id=4304061, core excerpt = "I want to subscribe to that person on a "coding" stream, and leave the "personal" stream alone. People are multi-dimensional beings with many orthogonal interests; Consider giving them multiple stdouts.") and App.net PM said it was duly noted. I.e. it will be ignored, and the site is destined to be a noisy mess just like everything else that currently exists, except it will also cost money.
Subjot (http://subjot.com/) would allow you to subscribe to people based on topic, but they shut down recently.