Nevertheless, I think this problem can be solved by using something like smartcards, which store the private keys and are able to sign transactions (with prior decryption through a passphrase). So the private keys never leave the smart card. Combine this with some kind of dedicated smart card interface-device, that displays the to-be-signed transaction and one should be reasonably safe. This concept is not new at all, but unfortunately not availiable for bitcoin, yet. Expect huge price jumps, if it is announced!