Unless you're offering Jeopardy style prizes (or at least something worth paying attention to), I don't see how you'd get particularly well qualified answers in a short time period. Expertise isn't cheap, and expertise on call is downright expensive (when I was a consultant I charged an annual service fee just for the privilege of calling my cell phone number outside of business hours)...so if someone can make $100+/hour consulting with their expertise, or make $10 every few hours via your service, they're obviously going to spend more time consulting than sitting in front of their mailbox waiting for questions that match their expertise.

I'm not saying it can't be done effectively and profitably--it probably can. We used SitePoint for a logo contest recently and had a half dozen professional designers, and another couple dozen serious amateurs, who responded over the course of 10 days...the prize was $500, though, so not chump change. We were extremely happy with the resulting logo, and we all (designers included) had a lot of fun with the contest.

But you might consider focusing on a few niches to get started, and actually line up experts on retainer just to prove to people that you have a working system (we've seen Google, Yahoo, and a dozen also-rans come and go in this field...most people are not going to believe that a startup can make something work that all of the big names in internet tech have failed to deliver). I believe you're going to have to prove it, but I might be wrong...maybe you can just let it run and gather momentum slowly.

The other problem is that most of the high end questions and answers I've seen (the ones where you'll make your money) on similar services have been extremely vague and, in result, poorly answered. I got a strong feeling that the questioner probably wasn't going to be very satisfied with the answers. Quality is historically very low from these kinds of services, and it's hard for me to imagine how you'd solve that problem without a lot of (expensive) oversight.