I take my D90 everywhere and before that, I took my Alpha everywhere. I usually just have it ready to go in the trunk of the car. I am a very impulsive person. Almost all the cool stuff I do starts like this...

... at a red light, I turn to my wife and say "Wanna go to the Monterey Aquarium.
"Sure"
"How fast can the kids be ready?"
"Ten minutes"
"Okay. Want a rent a luxury car?"
"Like a Cadillac."
"Nah, a hummer."
"Okay, drop me off and go get the kids ready."

So, knowing me and that nothing is planned, if I want good pictures I need to have the bag with me. I have one of those convertible bags that don't really look like camera bags and then all these straps pop out and it straps down over shoulder and belt. Very comfortable. I am also 6'2" and 250 lbs. and I generally don't have a "come and mug me" look. People usually think twice because of my size.

I say, take the camera everywhere. I do have a point and shoot that is beat up as all hell that I will take into buildings with me. Even if I am not supposed to have a camera, I usually do -- at least a point and shoot. I'll try and smuggle it.