A mile-crunching friend of mine swears by his Joe Rocker 'Alter Ego' jacket. Converts from warm weather to cold weather ridin easily ,and even has storage for its own extra parts. It has 3 or 4 different convigurations, IIRC.

I have been really happy with my Joe Rocket Phoenix jacket.  It's like wearing a screen door with armor in the summer.  Except the screen is really soft and comfortable and looks good on you.  If you can comprehend this then you know i like the jacket.  

A couple of my dirt-bike enthusiast mates have a sort of string/mesh T-shirt affair with armour in the crucial places. It can be worn in combination with just about any jacket or shirt.

Icon Super Duty
just remove the removeable sleeves

Doesn't matter what you wear, just make sure you have some kind of protection. I've been down before and I'm sure anyone else that has would also agree. My jacket has vents in sleeves and in the back. I wear it in hot arkansas weather (100 to 110 degrees sometimes), with the vents open, it's like a/c. This jacket faired pretty well in a 50 MPH lowside that I had.