I heard once that if an agent doesn't respond to you right away, it means she's not interested. An agent would jump on a good book as soon as possible to keep anyone else from getting it.
I heard in another place that if an agent doesn't respond right away, it means she's seriously considering the book. Taking on a new client is a big commitment, and it involves a lot of thought.
If the agent doesn't like a book, there's nothing to keep her from responding immediately.
You're killing me, guys. You're killing me!