Another vote for the PI Vagabond. It has back vents and you can unzip in the front to get air moving. When you get hot, you can unzip the sleeves and you have a vest.

I use a base layer, a jersey, arm warmers, and the Vagabond. That way, I can slowly remove or add pieces.

However, the Vagabond is not a jacket that will stand up to frequent washings. I had one for about 2 years and the stitching started to come out in 3 different places and the interior mesh liner came unattached at one point. (btw - I'm not the person who reviewed the jacket on Performance.com, so there are at least 2 of us who have had this problem).

Does anyone else know of a good, REALLY lightweight jacket like the Vagabond that also has venting and removable sleeves? I checked out a Gore Jacket with removable sleeves, but it was really heavy, almost like a wetsuit, compared to the Vagabond, which is like a really thin windbreaker.