I love my Kokatat kayaking shirt. It's vented like crazy, and I'm actually cooler wearing it than not. It seems to scoop air as I move. Flaps a bit on the bike, but I don't burn and I don't wear sunscreen under it.

http://kokatat.com/product_detail.asp?code=wdsh I think I bought mine at a kayaking shop, but I've seen Kokatat stuff at REI as well.