Since I want to be always fair, it ought to be noted that being a shirtless runner has its occasional negatives.
1) I've received everything from unmistakeably disapproving raised eyebrows to schoolgirls giggling to each other; anything from blaring car horns to catcalls and yells of "Put a shirt on, mate" could be directed your way.
2) If you do without a 'safety shirt', it isn't really convenient to drop by the corner store on the way home for a few supplies or the morning paper.
3) There are still people that I would rather not encounter during my shirtless runs. Certain old friends and the like. Irrational? If you like. But most folks have their 'hope not' lists, I suspect.
4) If you get used to running without a shirt, it's rather constricting to have to plod along in some form of upper-body wear. So when it's too cold to do without any, one is caught in a fitness 'no-man's-land'.
5) Gyms typically require patrons to wear at least a tank-top, so I've mostly given up on them since I don't do weights and prefer the outdoors anyway. But for those who prefer treadmills and the like, the problem looks pretty insuperable.
6) Depending on the length of run and the weather, the shirtless runner really mustn't forget his sunscreen (thanks to respondents on community.active.com for reminders on this point).
May 2015: First up, though I still try to put up blog content whenever I can, it has been easier to more regularly visit the the Twitterverse. Follow me at @barethomas10 and let's keep the shirtless running flag flying. Of course, the blog still attracts very interesting comments, and good discussion. Keep it up.
Second, in the years since this venture launched, and as shirtless running among women has gone increasingly mainstream, the term "jogbra" has clearly declined in use. I will thus prefer "sportsbra" henceforth - as has already been the case on Twitter, and in recent posts here.
I continue to welcome guest posts (sent to firstname.lastname@example.org) on any related topic, including from those who would discourage stripping to the waist. I am myself of course a fervent convert to the joys of running bare. But let all voices be heard!