I have been having the same problems with OS X 10.5.2 and Arduino0010. I have tried all the remedies suggested by the message boards and from what I've found on google. I'm installing the newest verison of the FTDI drivers for os x, have repaired permissions, I've deleted the kext from /System/Library/Extensions. The odd thing is that it worked fine when I first installed it, then I tried to load a program onto the board, and then it never worked again after that. I'm getting to the point where I'm going to reinstall 10.5 again. But would rather fix the problem, or create a patch if possible.
And ideas for troubleshooting/debugging this problem? Thanks in advance. You probably know this, but if you install a new FTDI driver, or remove one by deleting the kext from /System/Library/Extensions, you should reboot your machine to make sure the changes take effect cleanly. If you're comfortable deleting the kext, then I would start from scratch by deleting the FTDI kext, restarting your machine, reinstalling the FTDI driver and restarting again. After the drivers are installed, connect the Arduino to your computer with the USB cable - this should cause the driver to be loaded and the usbserial device to be created.