I find it interesting how different photographers use ISO based on their camera choice and experience. Specifically: film vs digital. In fact, while cmason and I are saying the same thing, we both approached this differently.
When shooting with film, you are basically locked to the ISO of the film. Sure, you could rewind a given roll, use a leader retriever, and swap rolls whenever you want, but most of the time and for most people that's simply not practical. You choose your film before a given shoot based on what you expect circumstances to be. There are many characteristics of each film that would make people choose one over the other, but for the sake of discussion lets say only ISO is considered. If I'm going to be outside at a family picnic I will likely choose a different film than if I'm in a dark house, or if I'm setting out on a hike intending to use the tripod for everything. I would always carry a small variety of film with me so that I could switch from 400 to 800 if I found I needed to (though I'd likely only switch after exposing the entire roll). Getting some ISO 1600 or 3200 film was for special circumstances and I wouldn't normally have that in my refrigerator ready for use. (Though on the other end of the spectrum I always had ISO 50 Velvia on-hand!)
So, yes, shooting film means I typically picked ISO first. Shutter speed and aperture were chosen later to make the photo. If the film ISO was too high or low, I relied on shutter and aperture to help me get a shot -- perhaps using f22 or even pulling out the tripod for something quick.
With digital it's easy to switch ISO. Often just as easy as setting aperture or shutter speed. I find that I do switch ISO far more frequently than I ever did with film -- and that's a huge advantage, for sure. But still, when I pull the camera out of the bag the first thing I check is ISO and I set it to what I think is a reasonable starting point. Headed outside on a bright day -- no point in ISO 1600, or even 800; 200 is more than plenty. Taking some no-flash indoor photos -- bump it up to ISO 1600 or maybe even 3200. I often work fast (typically chasing a 3-year-old) and find myself working the same old way as with film: setting ISO to something reasonable and then making use of the aperture and shutter speeds to get the shot.