CO2 Expeller asked a very similar question last week, but suggested perhaps the decline was indicative of a decrease in public concern about global warming. Wishful thinking on his part, perhaps.
My answer to him was that it would take further investigation to know the cause. You'd have to get a large sample size and determine how much the question rate has decreased, how much of the decrease comes from deniers, how much is due to new askers vs. the group of frequent participants in this section, etc.
I have noticed the decline, and like others my first guess is that it's due primarily to kids being on summer break. I have noticed a definite decrease in the number of denier questions, with the exception of Ottawa Mike, except that every so often there's a spike where deniers ask a bunch of questions all at once.
It's hard to explain. Perhaps the deniers burned themselves out on Climategate, and don't really have any 'exciting' talking points anymore. It's also northern hemisphere summer, so there isn't any cold weather for them to get excited about and claim disproves global warming. Perhaps it's because there's a bit of a lull in the US efforts to enact a climate bill. Perhaps the deniers are growing tired of being embarrassed in their Q&As on this site. Perhaps a combination of all these factors and more.
It may not be a coincidence that traffic on certain denier websites is also on the decline.
http://climateprogress.org/2010/06/23/blacklist-peak-readership-for-denier-blogs/
Whatever the reason, it wasn't long ago that I would look at the questions in this section and despite a page or two full of new questions, couldn't find one worth answering. It's nice that those days are over.
*edit* sadly there are a lot of Americans who think like Tomcat. We were headed towards an economic depression, which the stimulus prevented. But because we're still in a recession, they think it didn't accomplish anything. And they don't care that Bush created a massive deficit for no reason, but think President Obama is doing a terrible job for creating a deficit to stave of an economic depression.
Fortunately his claim that climate legislation isn't making progress is wrong. Democrats are rallying, they just haven't decided precisely which legislation to rally behind yet. But it's coming, probably this month.