I've installed the supposed updated driver from Sorceforge but no worky.

kextstat -b nl

.bjaelectronics.driver.PL2303 is empty

also 

Kext nl.bjaelectronics.driver.PL2303 not found for unload request.
Kext net.sourceforge.osx-pl2303 not found for unload request.
Kext com.prolific.driver.PL2303 not found for unload r

Any ideas?