There's a write-up on Ars about hacking. Notice how Blizzard blamed it all on keyloggers and viruses despite there being no evidence this was actually the case. And when he asked them to look into they said they couldn't. More interestingly Blizzard seems to be unable to track some logins, so they're sitting on a security nightmare. There are plenty of cases where accounts are hacked remotely without compromising a computer, as was suggested here. Not that anyone here will accept they could be wrong or acknowledge that their accusations were pure speculation.
If Blizzard simply uniquely identified every item and could track them all then this wouldn't be an issue but allowing players to drop items for others is just asking for trouble.