Bloglines, a Web based aggregator, displays subscriber #'s along with a feed. I don't know if you can get those numbers in a programmatic fashion, but Richard MacManus raises some interesting questions about how they might be used.
Predictably it's the thought of yet another ranking/indexing scheme for weblogs. Not that I'm particularly against them, but maybe one day new types of metrics and measurements. Stuff that would truly get at measuring influence.
Here's one off the top of my head. Think of each item published by a source as an impulse into the media network. Try to monitor the spread of that impulse, either through direct linking and/or trackback analysis. Evaluate sequences of items to come up with a moving average of "impact". This combination of measurements can reflect being in "the right place" in the network, centrality to network analysts, despite having few subscribers. It also dampens one hit wonders.
Of course having lots of subscribers can help you be connected to, and maybe even in, those right network places. But low flyers and emerging stars could still get their just due.