I'm with Chris on that one. Make one first, then ask, and if they're cool with it, make a portrait to go with it! You never know which you'll like better, when you get the film back.
More or less, I have a hard time approaching people for things like this, so I won't generally ask unless the photo itself seems to merit it, and when I do, it'll usually be after. I have this weird fixation with photographing the world as it is, as if there wasn't an annoying photographer there with a camera.
But first and foremost, respect for your subjects is necessary. Just try not to make bad photographs of people, and there shouldn't be a problem! If it looks like they're doing something, or thinking something, that no one else needs to see, don't trip the shutter. That's just sort of how I look at it -- even though they're in public, what right do we have to invade their privacy?
I just have a different definition of 'privacy' than non-photogs (and some photographers!)