http://en.wikipedia.org/wiki/PETNFrom reading the Wikipedia page on this stuff it seems to be fairly common. It seems reasonable to assume that this will continue to be used in similar attacks as the Shoe bomber and the Crotch bomber failed to execute properly. It also seems reasonable that we will not be able to detect this substance using conventional methods.
The only suggestion I have to try and control the use of this is to use bomb sniffing dogs at airports along with the metal detectors. This seems better than not allowing any liquids on the plane. If you want to take something on the plane you would need to open it up and set it on a table where the dog can walk past it and get a whiff.
Is there some reason TSA is willing to spend money on huge X-ray machines where you still need to depend on the operator being alert enough to see the liquid concealed on someone but they are not willing to spend money to acquire and train a few thousand bomb sniffing dogs to help keep airline passengers safe?