This is achieved by making a transaction between two wallets you control - this is recorded on the blockchain. Karpeles / MTGox did this previously to prove tha

This is achieved by making a transaction between two wallets you control - this is recorded on the blockchain. Karpeles / MTGox did this previously to prove that they had a certain amount of BTC. see https://bitcointalk.org/index.php?topic=21436.0 https://bitcointalk.org/index.php?topic=21436.0 for more details (search for 424242).