I have just purchased a Joe Rocket Phoenix Jacket and its awsome in the summer weather!!
I never have worn only t-shirts and shorts to ride, but this is a better alternative.
with this jacket it feels like your not wearing a jacket at all, but still there is all the armor padding.
I recommend this jacket to everyone for summer riding only.
I rode for 2 straight hours today in the hot sun and in moderate traffic, and I didn't even break a sweat.
Something to look into!
I got mine at Pacific Yamaha for $205 tax included.