A: This^, if you choose to draw and fire on the bike, you're going to have to stop the bike completely (meaning engaging neutral), draw and fire, plus probably end up dropping the bike, VS using the speed and maneuverability of a motorcycle to evade or disengage the threat. This translate into: On the bike = primary defensive mechanism is the bike, off the bike = switch to normal carry options. So having the gun on your person quickly accessible while riding isn't a primary concern.
B: Riding a motorcycle, you're far more likely to suffer an accident of some kind that you are to be attacked while on the bike. Dress for the crash. A bike will break your pelvis enough as it is, and wearing an unforgiving 3-5 lb chunk of metal strapped on top of that wont help, not that it would matter that much either way. Maybe its because I spent my adolescence growing up in Las Vegas, but try to play the best odds. If someone is more concerned with bringing a gun with them on the bike than a helmet and proper riding gear, then one could question their logic. If you look at it this way, odds of dying in a motorcycle crash (depending on what study you read, or believe for that matter) is about 60 in 100,00 (that's from NHTSA) . Odds of dying from a homicide is about 4.7 in 100,00 (from FBI, that's for all Americans, so probably nowhere near that high on the mean streets of Colorado), however I'd guess that riding on Colorado's sand covered roads make a crash slightly more likely. Now most homicides are the result of someone you know, so unless someone you know is out to get you, and specifically while you're on your bike, your probably pretty safe. Not to mention they'd probably just run you over, less hassle and clean up,

. There are no homicide rates specific to victim was riding on motorcycle at time of assault that I can find, but I'd imagine the rate is pretty damn low. BTW: the NHTSA injury rate per 100,00 for motorcycle riders is about 440.
I guess my whole point is focus on the riding part while riding, leave all the other shit for when the kickstand is down.
But to answer your question in short, I carry a little snubnose in my pocket. I can't even get to it on the bike due to the gloves and the angle of my leg (riding a Yamaha FZ-09 as of current) it's just there for when I get off the bike. If I'm going to Denver or for an extended stay somewhere I'll carry normal gear in my backpack. (Have a hard spine protector in the backpack between me and all that shit, plus the normal articulated spike armor under my jacket.
As for the guy that mentioned the shoulder holster, I'm not knocking you or anything, but that makes my ribs hurt just thinking about taking a hard tumble with a pistol in my ribs. To quote the great Bevis and Butthead "No way dude"