I finally found out what the blue-butterfly car decals mean, and now I know why most of you didn't know what I was talking about: It's a local thing. Matt told me that his dad's girlfriend's daughter Ashley has one on her car, so I emailed her, and here's her response:
"If you look closely at the decal, the body of the butterfly is shaped like an 'S,' the top half of the wings makes an 'M' and the bottom half of the wings makes a 'W.' 'SMW' stands for Sara Michelle Wing, Eugenie Wing's daughter (she owns the pottery shop). If you don't remember, that's the girl who was killed in a car accident in 2004. She was a really sweet girl and her friends designed that decal in memory of her."
I'm going to try to take a picture of one to post here so you guys can see what they look like, and then I'll let this drop. Thank you, Ashley, for helping me cross off No. 1!
Update: I finally got a picture! Ta-da: