Sounds like material is not the concern. Aluminum tends to be lighter. Steel tends to warn before it breaks, and can be repaired in the boondocks. Both are equally reliable on a well built/well designed bike, and a good bike will be highly unlikely to fail unless you are in an accident. Both also oxidize, but aluminum less quickly than steel. A high end bike will be treated on the bare surfaces on the inside, but any bike can be treated to prevent rust.
Unless you are OK with a singulator/chain tensioner, you probably want a bike designed with stationary chain stays. That leaves out the Brompton and Birdy, which are two of the easiest bikes to pack in a suitcase and the two that are among the best built bikes. (Though the Brompton is too uncomfortable for most as a touring bike.) It also mostly leaves out Bike Fridays, though there may be a model without a hinge between the bottom bracket and the chainstay (??).
The Swift is probably the best fixie bike, but the most difficult to pack, and cannot be packed into an airline legal suitcase or anything close without complete disassembly, I hear tell. That, once again, leaves the Dahon. The 2008 Mu models look to be winners, with improved frame hinges and stems, but Dahon has a long history of less than perfectly reliable hinges. The Mu SL is not going to be a good touring bike given the lack of spokes in the wheels (one branch in there and you are done with your tour). I would also highly suggest that you have Gaerlan build your bike for you. I bought my Birdy from him and had him build my wheels and he has always done an exceptional job. He will also tension the spokes, another problem associated with the factory built Dahons.
Bike Friday is of course the king of custom built folders. But I have yet to talk to someone who has been able to fit one in an airline legal suitcase without major disassembly. Like the Dahon, you will probably need to go with a larger 31"-33" suitcase and hope for the best at the check in counter.
Even my Birdy requires removal of the wheels, front spring, seatpost, and one pedal to fit in a 29" suitcase. It is probably one of the best for touring, with a high quality frame build, an outstanding ride, great luggage capacity, and easy packability, but a *very* bad choice for a fixie. It uses standard parts, but any slight deviations from the precise factory build causes problems (e.g., a chainline that is slightly off spec. The spec chainline would be terrible on a fixie, and you would drop the chain every time it was folded if you deviated.)
In sum, I would consider a 31" or larger suitcase a new Dahon Mu P8 when they come out. If you are a risk taker at the check in, your very best bet would be a Swift and a ginormous suitcase. If you are wealthy, you could also have the most of all worlds (aluminum, fixed chainstay, packable, fast) and get a Moulton Bridgestone for $2K (available only in Berkeley, UK, Japan), but it would not fold. I have found that folding is critical on tours unless you also want to pack a 20 pound Kryponite chain.