As a side note, in the airport I saw a medical dog of some sort that was wearing a harness, but then the harness had a patch that said "ask to pet me, I'm friendly". I thought that was crazy, talk about sending mixed messages!
Actually, it's kind of brilliant. "Ask to pet me" reminds people that just running up and petting a service animal is not okay. That tag probably reduces the number of faux pas in that area.