First off, well done for doing the right thing, and you've earned my respect.
But here's a question.
The passcode. I'm pretty sure this isn't new as of 2.2, but if you mess up on the passcode 10 times, and the iPhone wipes everything, would that include the Contacts as well, let alone the passcode?
Assuming that it does, you could get the phone unlocked (read: no passcode), get the number, take it to Optus, and they find the person. They'll get back basically a vanilla iPhone and have to enter in their contacts again; small price to pay, but they'll have their phone back (if they're smart, they'll have those somewhere else). Apps/music/video/podcasts/etc. are easy to get back..
Outside of that, the others are pretty much spot on. AppleCare may be able to help, since the phone is still under warranty.. Could Optus drop the SIM card into another iPhone and get the number, drop it back into the iPhone you found, and take it from there?
BL.