GLC, I have it, too. Usually just in the first couple of long rides in the season when it's sunny. I've been thinking it's a heat rash. One of my cycling friends has it, too. I don't think it's a contact dermititis--like you said--it doesn't matter if the shorts are long or short. It's always in the same place. Funny, though, I never feel like the tops of my legs feel any hotter than anywhere else.

We'll have to call it "Ugly, strange, cycling rash!"