I've been chewing this over in my mind a bit today, I've been following the discussion (somewhat) both here and on bikejournal. A lot of people would consider T-shirts like this (and anti-cyclist groups on Facebook or similar) harmless fun. Edgy and provocative, but still harmless fun.
And to some extent I agree. We all get irrationally pissed off at things sometimes, and I prefer when people visibly show it rather than pretending that everything's fine. I don't mind when people get really riled up about everyday things, as long as they can laugh a bit at themselves too.
But "harmless fun" means jabbing at someone who's your equal. It might be fun to threaten to run a cycling buddy off the track and into the undergrowth when you're riding a bike, not so fun to do so when you're in a motorized vehicle capable of crushing his skull. I don't think drivers get how vulnerable cyclists feel out there, at all. And how we EVERY SINGLE DAY have to consider how to NOT get hit by a car. What looks like a "harmless" violent fantasy for them is stark reality for us, and just not fun.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett