meffoos, I've heard that about margin accounts before, and right now my Ameritrade account is margin-enabled, but the shares haven't been deposited into it yet, at least as far as I can tell. They appear in Gainskeeper but not within my account itself.
So even if the brokers can short shares within a margin account, there are no Paivis shares in my margin account, not yet at least -- unless they have received the certs and are shorting the shares before reflecting their delivery in my account. That would be illegal, I would imagine, although I can't see any way to prove that's what they're doing.
Something has to explain where the shares being sold right now are coming from.