CASS is the system that emulates the good ol days, pre 9-11, where for instance an ASA guy could ride IN THE COCKPIT of a United jet, or FedEX could ride up front in Northwest. It's basically an electonic verification that the airline can use to look up if you're who you say you are. Some airlines are on the list, some aren't; it costs money to run it, and not all airlines want to allocate the funds.
I understand that the TSA has just recently taken over the whole program (from AirINC, i believe). I've been told that this should make the process easier, but considering it's the TSA, I doubt it. These guys are too busy x-raying their heads to see what their brains look like.