I never knew about this either, it was like a freak thing that I ever brought it up to the vet.
We had gone down by the river to play with a friend of mine and her 2 pits. After an awesome day of fun in the river (and the sun) I got Roxy home and she just passed out. Well I had to work that night and my girlfriend said Roxy was crying allllllll night. Of course, I was flattered that she would miss her Mom so much, but it didn't stop. The next day every single time she woke up from a nap she was crying (almost screaming) in pain. So off to the vet we go. She had freaking sunburn. Who the hell knew. I mentioned the hives she gets, and now we are ordering enough accessories to officially call my dog a ginger.
She LOVES the sun, completely obsessed and I feel horrible that she has to stay out of it. So I put some children's SPF50 on her today (as per the vet any sunscreen made for kids that is UVA & UVB) and let her go lay in the sun for 10 minutes. The hives were INSANE. Worse then they have ever been, some were actually blotched, raised areas and not just hives. I don't think her skin likes human sunscreen, so I ordered some Epi-pet sunscreen (their entire line of products got great reviews). I feel horrible that I have to keep her in the house in such beautiful weather, but, it is better than skin cancer down the road.
Oh, one more thing while I'm ranting....whenever you see your pup get the hives, take pictures! That way the next visit to the vet they have those photos. I hope all ends up well with everyone.
Also, here is a link for the sunshirts (because it took me a while to find them, and there seems to be only one brand. I will post pics of Roxy sporting them whenever they come in)http://www.amazon.com/gp/product/B0069UUMBM/ref=oh_details_o00_s00_i00