Well anything can fail (ask PatentCD). However, for longevity Ti is really your best bet. It does not rust, and will not fatigue ( i.e you can bend Ti within its limits over and over with no loss of strength, unlike AL, which eventually fails from fatigue). Also Ti has a greater resiliency than steel (i.e. you can bend it farther under more force without permantly deforming it. Doesn't mean a weld on a Ti frame can't fail, and doesn't mean that you can't wreck it so bad that it doesn't bounce back, but on average Ti frames will last the longest of anything currently being built. I rode last night with a guy with a 1991 Merlin that still looked new after 60,000 miles.

Only longevity argument in favor of steel is repairablity. If you trash a tube on a lugged frame (particularly a silver brazed lugged frame, its not a big job to pop in a new tube. Waterford did this on my Paramount OS when the seat tube snapped off at the BB.