I have a Castelli "Simple" jersey that's lighter than all the other jerseys I have. The material has a mesh-like design and I suppose in hot weather it'll wick more efficiently and circulate air better. (I don't wear it much because over here in San Francisco it's usually kind of windy and I get cold easily so I prefer jerseys with a tight knit.) It's a bit pricey but I've seen the previous year's inventory sold at a good discount.

-Kevin