/dev/tty.usbmodem disappears after Mountain Lion upgrade
I beleive that plugging the arduino into the Mac via USB causes the port to be created automatically as part of the device detection.

If it's not showing up, then the OS is not detecting it as a device or the name of the device is no longer waht you expect it to be.

It may well be that under Mountain Lion, that the port name has changed, I found a script over on StackOverflow that detects for changes in the /dev folder by comparing a list of all the devices in /dev before and after the device is plugged in.