FFFlyer - there are indeed various detection systems that will pick up explosives such as C4, Semtex, PETN, NG, RDX, EGDN etc in checked baggage through the use of scatter radiation.
The main problem is that these systems are very expensive (when they first came out they were well over US$1m each!) and though most major European airports are equipped with them from what I understand coverage is very sparse in the USA and generally limited to major international gateways.
Hopefully, as demand picks up prices will drop making them more widespread.
There are also vapour detectors in use that will pick up passengers carrying such items on their person or in their handbaggage.
Incidentally, the reason for the 'Samsonite ban' was simply that hardsided cases such as the Samsonite and Delsey permitted the installation of hidden compartments and/or the use of existing cavities for the carriage of explosives or other contrabrand. They were therefore given a more thorough examination, though I don't think there was ever a ban per se - most of the world's airline crews use them!