I read about others with this, but I heard it with with OS 10.4. Anyway, fixes I have read on this forum are to hold reset while plugging in the board or to change from tty to cu and output verbose.
My board is an UNO (got it today). At first, network pref would say it found a modem and I would hit "apply." However, after installing the FTDI drivers as a troubleshooting step and restarting my computer, net pref no longer see's arduino (yet I can still loopback).
Binary sketch size: 1018 bytes (of a 32256 byte maximum) /Applications/Arduino.app/Contents/Resources/Java/hardware/tools/avr/bin/avrdude -C/Applications/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf -v -v -v -v -patmega328p -cstk500v1 -P/dev/tty.usbmodemfd321 -b115200 -D -Uflash:w:/var/folders/ra/raeYEsvpFsGnJ2Edp3XO8k+++TI/-Tmp-/build237537096815304598.tmp/Blink.cpp.hex:i avrdude: Version 5.4-arduino, compiled on Oct 9 2007 at 11:20:31 Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/ System wide configuration file is "/Applications/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf" User configuration file is "/Users/grantgustafson/.avrduderc" User configuration file does not exist or is not a regular file, skipping Using Port : /dev/tty.usbmodemfd321 Using Programmer : stk500v1 Overriding Baud Rate : 115200 avrdude: Send: 0   avrdude: Send: 0   avrdude: Send: 0   avrdude: ser_recv(): programmer is not responding avrdude: stk500_recv(): programmer is not responding avrdude: Send: Q   avrdude: ser_recv(): programmer is not responding avrdude: stk500_recv(): programmer is not responding
I performed a loopback test (which I read about here) and that succeeded.
Any help is appreciated!