Hi,
I'm running 10.5.2. on a PPC machine; I also needed to dl the FTDI driver. Now it's working and I see and select /dev/tty-usbserial.... After reading your messages, I hope it will remain this way :-X
what kind of macs are you using with 10.5.2? I noticed that there are two drivers on the FTDI driver download page, the (Intel)-one requires 10.4. ! Maybe that's the reason it stopped working?
btw:
it's surely a good idea to restart after installing a kernel extension, but not really necessary: Just open the terminal and type: sudo kextload /System/Library/Extensions/FTDIUSBSerialDriver.kext
you will be prompted for your admin-pw and then the new driver will be loaded (hopefully). If not, check if the path to the file is correct!
The same can be done to unload extensions with kextunload.
Best,
Michael