I have an Eleven81 rain cape that works great in cooler weather however during warmer temperatures it can still retain heat and cause me to sweat... but it's far better than a even a breathable (GoreTex) rain jacket at all temperatures if sweat is to be kept at a minimum. It also offers good lap protection and even lower leg protection if the wind or your speed isn't too great. The downside is that you cannot wear a backpack over it and unless your backpack is small/slim, it won't fit well under it either.
Brooks offers a very nice waxed cotton cape that I've tried and liked. Very nice indeed (I'm a fan of waxed cotton) but also very spendy.
http://www.bikepedia.com/pa/Item.asp...6#.UCw4cHi21UQ
BTW UMassAm (are you a Zoo-Mass student or faculty member?

), I'm in the process of running Nikwax through the paces.... you may want to give it a try on anything you have as a water-repellent shell. I've even treated a pair of cotton gloves with the stuff and so far it beads up water like a duck's back.
More info.