Here is what I found. The driver is the issue with Yosemite.  The built-in FTDI driver prevents the osx-pl2303 driver from loading.  The fix (not for the faint of heart) is to do some black magic at the command line.

In Terminal, enter:

sudo nvram boot-args="kext-dev-mode=1"

This allows other (unsigned) drivers to load.  The driver that works for me is the Lion driver from miklor.com.

Then manually load the driver (first time only,  it will load automatically after that) with this command in Terminal:

sudo kextload /System/Library/Extensions/osx-pl2303.kext

This creates the device cu.PL2303-000012FD, be sure that is what you select in the CHIRP Radio dialog in the Port drop-down. Again in the Terminal, do "cd /dev", and then "ls" (without the quotes) to  see that the cu.PL2303-000012FD driver is actually loaded.