Ok, so I tested this. it appears that as long as I plug it into the same port, I get the same value.
[u]Usually[/u]. Using a Keyspan adapter in another application, I had an incident where the name actually changed between reboots (a "1" was prepended for each reboot!). I do not know if this is a function of the USB enumeration of Mac os X or the Keyspan driver, but it wouldn't surprise me to see it happen with other devices.
So then there is no way to predetermine the port.
Not exactly. An FTDI installation guide for Mac os X says:
If the device is installed properly, you will see entries in the /dev directory:
where xxxxxxxx is either the device's serial number or, for unserialized devices, a location string that depends on which USB port your device is connected to.
So, if the device has a serial number, you get a unique name no matter where you plug it in (or which system, for that matter). If the device does not have a serial number, you get the behavior that you are seeing.
I got my arduino from SparkFun and it was serialized, so I didn't get the annoying behavior that you are seeing. I do not know if all USB arduinos are supposed to be serialized, or if just the SparkFun guys do so, or maybe yours slipped through the cracks.
FTDI supplies a utility to program the EEPROM in their devices. If you are adventurous and have access to a Windows system (unfortunately the utility is windows only, and requires at least win2k IIRC) you can load the FTDI drivers and this app to add a serial number to your device. I had a different FTDI device (an rs232/USB adapter utilizing the same chip as the arduino) that had no serial number, and I successfully gave it a serial number and it immediately started behaving in the more desireable manner on the mac.
You could play some sort of game like
my-mac:~me$ sudo ln -s /dev/tty.usbserial-* /dev/tty.arduino
If you do this on every system, you should be able to just use /dev/tty.arduino all around (unless you get the strange reboot bahavior I referred to earlier). I have not tested this; YMMV.
It appears that .usbserial- is common to all of the ports.
Yep, that's what the manual and my experience both indicate.