Right, this is all rather intriguing but I'm rather perplexed by the lack of a username/password login thingy when I launch the client, seems all I get is a "main" bitcoin address and a receiving address, which seem to be identical.  I just don't get how people can't just use my address to not just send money to it but also pretend to be me?   I just don't understand it how the whole thing works.