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.
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. Then, in a terminal window, type
ls /dev/*usb*
to verify that there's a tty.usbserial device. If there isn't, check your system logs for error messages. The following command lets you do that conveniently
bzgrep FTDI /var/log/system.log*
If things are working, then, after connecting the Arduino, the bzgrep command should reveal a message like
/var/log/system.log:Feb 16 07:57:17 teresa kernel[0]: FTDIUSBSerialDriver: 0 4036001 start - ok
If there was a problem loading the driver, I'd expect the system log to contain an error message, instead.
If things still don't work, here's a little shell script you can use to gather information that you can post here on the forums. It includes the commands suggested above and also extracts version information from the kext. To use it, copy the text into a file (like, "ftdi.sh") and then run it by saying "sh ftdi.sh".
#!/bin/sh
#
# Report information about the FTDI USB serial driver
#
echo "[Extensions]"
for KEXT in `ls -d /System/Library/Extensions/*FTDI*` ; do
echo $KEXT
find "$KEXT" -name InfoPlist.strings -exec cat {} \;
done
echo "[Devices]"
ls /dev/*usb*
echo "[System logs]"
bzgrep FTDI /var/log/system.log*
FWIW, I've been using Arduino 0100 successfully with a Diecimila and an old G4 Powerbook under MacOSX 10.4 and 10.5 (now 10.5.2), both with the original FTDI drivers that ship with Arduino 0100 and the latest drivers from FTDI's website.