It's not the number of takeoffs and landings that fatigues the Al. It's the number of pressurization cycles.

Never had a frame break on tour, course I've only done three so far. I have, however, had two other frames break. Both were steel, both cracked at the right dropout/chainstay interface. I don't own any aluminum bikes, but I wouldn't hesitate to ride one on a tour. Especially for heavy riders like myself, a good stiff Cannondale touring is probably a better bet than say a trek 520, which would be noodly under so much load. My touring bike is a trek 930 MTB, which with its oversized frame tubes has superlative frame stiffness as good as aluminum.

And worrying about frames breaking on tour is just one more way to express paranoia about embarking on an big adventure. Hell, if an aluminum frame breaks on tour, you could probably macguyver it with fiberglass to get you through.