In Optimizing for Pearls, Not Sand, Jeff says that:
We now limit users (and IP addresses) to a maximum of 6 questions per day and 50 questions per month.
Is this a per-site limit or a per-network limit? It seems quite possible that a single IP address could generate more good questions that this across the wide range of Stack Exchange sites that exist.
For example, when working (at home) I have two good questions about programming and database management, then when I'm not working I have questions about the meal I'm planning to make, how to best photograph that meal for the blog post I'm going to write, and then late at night I need clarification about the rules of a game I'm playing. During the day, I also have a good idea about how to improve traffic at the beta site I'm involved with, and trigger a bug that I would like to report.
This may seem like a bit of stretch now, but assuming the number of Area51 graduates continues to increase, it will become less and less of one.