What's the chance one of my opponents has a flush draw?

[M.B.E.]

Here's a problem I've been wondering about for some time, with some major implications for correct holdem strategy.

Suppose I'm in the big blind with Ah-Jd and take the flop with four opponents. The flop is Js-7s-2d. What is the probability that at least one of my opponents has a flush draw?

Notice that I haven't specified the preflop action: whether there was a raise, whether the players are in early, middle, or late position, etc. My instinct is that this shouldn't make too much of a difference to the solution.

Obviously to answer this question you'll have to make some assumptions about what hands my opponents will play and what hands they will throw away preflop. Be sure to state your assumptions. Hopefully you can come up with assumptions that would reasonably approximate a typical holdem game (let's say the Mirage 20-40 if you need a reference point) but still make the problem easy to calculate. For example, you might say "assume my opponents will play anything in Groups I through V of the Sklansky hand rankings, and nothing else". Or if that turns out to be too complicated, you might say "assume my opponents will play any pocket pair, any suited ace, any suited connector 76 or bigger, and AK, AQ, AJ, AT, and KQ offsuit, and nothing else".

By the way, my guess is that whatever reasonable assumptions you make, the answer will turn out to be between 40 and 50%. One of the reasons I ask this question is that a lot of times when we're discussing hands on another forum, it's assumed that on a two-tone flop, you're probably up against a flush draw; I'd like to know whether that's correct.