The PX-850A and PX-850SA quietly showed up on Plextor's website this morning. The two drives support Plextor's new PlexUTILITIES software and are capable of 22x DVD±R, 8x DVD±R DL and 12x DVD-RAM writing speeds. The only apparent difference is that the PX-850SA is SATA.
PX-850A - http://www.plextor.com/english/products/PX-850A.html
PX-850SA - http://www.plextor.com/english/products/PX-850SA.html
If I had to guess, I'd say that these two drives are based on the Lite-On iHAP322 and iHAS322.