There's some interesting info on steel and AL frames on the Rivendell site. They don't make or sell AL frames, so there's always the chance that they're saying some of what they do to hype their product, but it can't be that far off the mark.

One thing that's always concerned me about AL is the "immediate and (possibly) catastrophic" failure of AL -once it begins to crack- compared to steel. I've never owned an AL frame long enough to really judge the ride quality, but I'd like a bit more warning than AL provides before a frame comes apart. A well-made, well-maintained steel frame can and should last a lifetime.