As has already been mentioned the 4785t requires BIOS 503 or later; the current latest is 601. It's possible that retail units are being distributed with the 0209 first release BIOS or at least one earlier than this CPU needs, and this could account for the non-booting. Is there anything on the box...

I have the exact same problem, H81T with 4785t and no boot or POST signal. I have tried both the external and internal audio jack with no POST signal. I have also tried to clear RTC with the jumper and disconnected the battery to re-set the bios without succes.

Have you had any succes?