By posting online, a bloke in New York got his friend's phone/PDA back.
I'll recap in case you haven't heard the story. A woman left her Sidekick in a taxi. (A Sidekick is a convergent device, combining mobile phone, pocket computer and camera.) This device was sold by the taxi driver (or another passenger), and ended up with someone who took pictures of herself, etc, not knowing that the phone company automatically backs up the owner's memory to a web site.
To cut a long story short, the owner's friend was able to identify the possessor, and requested it be returned. This was met with various responses, including physical threats and demands for money. He then started a web site on this, that got picked up all over the place. He requested people not hassle/threaten the possessor, just wanted the device returned. Many offers of help and legal advice, and media attention. Eventually the device was returned after the police charged the possessor with a misdemeanor.
The bloke had spent a fair bit of time and energy on this, largely on principle. Throughout, he tried to be fair, legal and honest. The possessor was aware she was in illegal possession, and that it was illegally bought. The NY police did not come out of this very well, because it took public pressure before they acted, despite having evidence of who had it and where it was.
Some things on the internet just take off. No guarantee of successful results for you. But it's an interesting example of what can be achieved.