Intriguing question. I personally consider Northwest and Delta to be the safest FF programs -- not including "upstarts" like Southwest, Jet Blue, and others, who don't have a long track record. This is primarly through the process of elimination (from worst to first):

UNITED: The shakiest program; from a financial point of view, the airline is in intensive care, with everyone is watching their vital signs regularly.

USAIR: Seems to have emerged from bankruptcy okay, but never really had a "psychological" commitment to frequent flyer programs. One of the last airlines to implement a FF program; if the programs start disappearing, USAir will probably happily lead the way.

AMERICAN: Narrowly avoided bankruptcy after 9/11; still a little shaky, but working its way back.

CONTINENTAL: The fact that Continental still exists as an independent airline today is one of the most amazing survival stories of our generation. Seems to be okay now, but risky due to past history.

NORTHWEST AND DELTA: Probably the financially healthiest of the major airlines. Unfortunately, this is because of their willingness to cut service and seat space in order to be financially competitive. (This may also save American, which recently ended its "More Room In Coach" program and started packing passengers in like matchsticks again.)

Just my two cents. What do others think?