Thanks everyone. The 2 tubes both had very small puntures in them. After the second failure is when I was able to correlate the location of the hole in the tube to the cut in the tire. Whatever caused it was probably still stuck there when I put in the second tube, causing the failure.

Beleive it or not, this is only my second (and third, I suppose) flat, ever. All of last year, I had only one flat when a car ran me off the road onto a gravel shoulder, which the side of the asphalt cut into the tube. Next time I'll definately be sure to inspect for FB more carefully.

I put a patch on the inside of the tire, installed a new tube and just did a few laps around the block without any problems. I'll keep an eye on it though. Thanks again.

EDIT:
After reviewing this picture I just took of it, I don't think I like the way it looks, and might order a replacement here.