Well I may've just avoided a disaster. I opened the closet my carboy is stashed in to check on the stout I've got in there, and I noticed an ant crawling across the top of the bottle. Sure enough, there was a cluster of them in the back corner and a trail going across the wall. I moved the carboy to another room and gave it a good look over, killing three or four ants that were on it. No signs of them on the inside, but I'm sure they wouldn't go through the water in the airlock, and I think the seal between the stopper and the bottle is probably too tight for them to get into anyway.
I took the bug spray to the closet, vacuumed up the dead, and all seems to be well now.
Though if some did get into the brew, I guess I could've marketed it as some kind of exotic gourmet brew. "Real ants in every bottle!"