(Or at least figured out how to fix it, anyway!)
Bloggers: If this should happen to you -- that an ad pops up when your blog address is accessed -- it's most likely because one of your gadgets has been infiltrated. I confirmed that this is what happened here when I removed all the gadgets from my sidebar -- and voila! We were back in business. Unfortunately, I can't tell you which gadget is (currently) infected because I couldn't view the blog to see what I was erasing; I was just rolled back right into the stoopid "bloggroll" ad when I tried to figure it out. But, so long as you can get into your Blog dashboard, you should be able to get in and remove all your html goodies -- which should get rid of the problem. If anyone finds out which one is the culprit, I'd love to know, though.