I've been exchanging bitcoins locally for a 6% spread: selling for 3% over Mt.Gox and buying for 3% under. You're right, there's not much money to be made when the value of gas and time and the exchange rate volatility risk is factored in, but I'm excited to be a part of monetary revolution, and I get to meet a lot of interesting people. Besides, I'm building trust at this point, and once the volume gets high enough, the profits will be there. You can also look at it like you would running an I2P router: you aren't making any money off the other people using your bandwidth, but you are improving your own anonymity in the process.