I think since the pages on my widescreen monitor are just blank white on the sidebars, there's room for an ad or two. But please integrate it with the users of this site. There are a lot of software engineers on here. Wouldn't it be great if they could advertise at a discounted rate on their favorite site. A site to which they have contributed many answers/questions and helped bolster (for free). Especially if the project / product they'd like to advertise is about mathematics itself.
Addendum: wouldn't it be great also if the Q&A pages that you advertise on, if the users who wrote the content on that page get a small cut of the ad revenue? That's the only fair way to go about this. If not, then I vote no ads.