ing in Florida I go jogging year round, and it is always shirtless. I travel 100% for business and always jog shirtless if the weather permits. It feels GREAT, there is no nasty/clammy/stinky shirt to deal with, and you set a great example for other guys... and get an occasional TOOT on the car horn too!
I wear regular nylon running shorts, like you find in running stores or on the cover of Runners World magazine.
http://www.Runnersworld.com. My current pair are bright yellow (for visibility) with a mesh liner. I carefully took out the side seam so they are now side-split to the waist, there is an overlap when standing but not when running or sitting.
I do NOT see how you can run in long shorts, or even work out at the gym, which I do 4x week. I would work out shirtless if the gym would allow.
http://www.NudeRuns.com lists all naked 5K runs around the world, which I have participated in several times... and also the Bay To Breakers 12K/7m run in San Francisco, actually the nude contingent called the "Bare 2 Breakers."
I say, set a positive example by going shirtless whenever possible, especially when exercising.
And don't get me started on guys doing the "towel dance" in the locker room because they are afraid someone might see their wee-wee. Dudes, we've ALL got one!
http://groups.yahoo.com/group/shirtless_lifestyle
Take care, chests bare!