I just bought the "Arduino Duemilanove ATmega328" and installed the Software "Arduino 15" and the USB Driver "FTDIUSBSerialDriver_v2_2_10.dmg". The Problem is: I can't find the driver in the menue!? My system: Mac OS-X, Intel MacBook Pro. I will receive the Board in a few Days :). Have a look at the screenshots.
Yup, the actual USB-serial device is on the Arduino. The way OS X does BSD style devices means that few of the "disconnect" devices will actually have a character device file reference.
Unlike, say, BSD or Linux, which will have /dev/ttyU* (or whatever) present but unconnected to anything so it looks like a TTY or modem that is just not there.