Thanks for the reply. I was one of the early adopters of this camera, having received mine in late November 2008. It took me a whole two months to realize I wanted another one, which I got in February 2009. Great cameras that do what I need, with no desire to upgrade anytime soon (though the AF of the MkIII makes it tempting!).
I'm actually away from home for a few days without my MkII's so that's why I though of asking here instead of testing for myself.
So Auto-ISO takes various exposure parameters into account and goes for the lowest. So I'm assuming that in Av mode, it factors in lens f/stop and focal length, and chooses the slowest possible shutter speed and therefore the lowest ISO for a good exposure (maybe choosing a shutter speed along the lines of 1/focal length, or even slower if IS is enabled?). Hmmm, I'll have to test it when I get home tomorrow night.
It would also be nice if there were a way to tell Auto-ISO to use increase ISO by a stop to force a faster shutter speed. Sometimes when shooting events, I purposely shoot wider than the final crop. It's just easier during fast action, or when shooting primes. Cropping in post would be similar to using a longer lens, so that's why I'd want a faster shutter speed.
Thanks for the response. If anyone else knows has answers, please share!