I ride a Rans V-rex with a mesh seatback, which logically does provide better ventilation than a solid seatback would. For what it's worth, I've never felt that it's any "sweatier" than my diamond-frame bike -- if anything it's cooler. The Rans has a pivoting handlebar stem, which allows me to occasionally straighten out my arms and get a cooling windblast up my sleeves and directly into my armpits -- quite refreshing on a hot day!
Lee