I've used a (non-Arduino) CDC device on the mac, no problems. My guess is that zterm is broken, or something else (like OS X networking) grabbed the "modem" before you did. I use kermit, and it worked fine.
USB CDC is "communications device class". Just as USB defines a standard for mass storage devices, so we don't need drivers for every single type of thumb drive, USB also defined an interface for rs232 serial type devices and called it CDC.
Why no vendors actually use CDC instead of sticking us with driver installs is beyond me. Maybe there's some basic design flaw in CDC, or maybe there are just lots of idiots in the world. I guess I shouldn't complain, since I like FTDI's driver better that the OS X CDC driver anyway.
-j