No way will it be approved for use while flying, or soon before. Once it becomes legal on the federal level, they will need to establish a "toke to yoke" policy similar to the present "bottle to throttle", as well as a hard limit to blood concentrations of at least THC and perhaps its metabolites as well.
How to regulate how much THC is automatically assumed to be under the influence is one of the thornier questions facing Vermont's legalization movement right now. Also, Gov. Shumlin has gone on the record as favoring a .05 limit for drivers found to have both alcohol and recent use of cannabis by blood testing, an interesting proposal. I'm not sure how solid yet the science is on determining by bloodwork whether a person can be presumed to be under the influence of THC, nor how long ago they ingested the drug. Opponents insist it can't be done yet, and therefore we'll have an unenforceable plague of stoned drivers if legalization goes through. Proponents insist we already have that plague and legalization will not likely make it significantly worse. But no one that I've heard has convincingly argued that we can reliably tell with a blood test when someone is stoned. (Of course, that's true of alcohol as well, due to differences in physiology that allow one person to be barely affected at .04, while another is visibly impaired at .02 or even less.)