In my experience most of the frames that come into bike shops with cracks are aluminum. I suspect that aluminum is somewhat more prone to cracking than steel. But I also suspect the increase in failure rate is so slight as to be worth ignoring.

A somewhat more pressing problem, IMO, is that aluminum tends to fail suddenly whereas steel tends to bend first. That makes me nervous, but again, is probably too small a concern to actually worry about. (Carbon, of course, fails suddenly and catastrophically too, so choosing carbon instead of aluminum because you're worried about cracks is stupid.)

Oh, and mprelaw? Airplanes are subject to stringent and thorough inspections because of aluminum's propensity to crack and fail. So that probably wasn't a very good analogy...