[wlphoenix]

I only have a vague memory/understanding the subject, but I believe the Bitcoin protocol supports the concept of multi-signature contracts. That could be used to create decentralized escrow/trading/exchanges, which means that you would never have to let a single entity control your private keys. (https://en.bitcoin.it/wiki/Contracts)

I don't think there is a working implementation of the multisig contracts in the wild yet, but the protocol does support a way to solve these problems.

[gnaritas]

That won't work for traders, transactions on the blockchain are too slow to do what exchanges really do. Traders would not use such an exchange, it'd be too slow.