I also have this Autoplay issue which I raised in a post here recently. I have spoken to Sony support (UK) who because I experienced it on my two computers, one with XP and one with Vista, have told me that it must be the PRS-600 and that it should be returned for service. I do not think for one moment that this is likely to be the cause because the "Launcher" partition on the PRS-600 is write protected and so the Autoplay which is in this partition cannot be modified. I had also completely reset the reader to factory state to no effect.
It seems more likely that it was because I had manually installed EBL 3.0 before I got the PRS-600 that something is left behind in registry etc?.
I will not return the reader to Sony as this just a nuisance and can be worked around but I would love to know how to solve it.