But looking at these sites again, I'm confused: they seem to have substantially different prices. E.g right now about 47.3 USD/btc at Mt.Gox, 46.1 at Bitstamp, and 44.9 at BTC-E. I must be missing something. Can anyone explain?
They are all different exchanges, unlike fiat where one exchange price rules the exchange, bitcoin has as many as the exchanges that there are. Currently Mt Gox, has the most volume so people use them as what the current exchange rate is. But nothing is wrong with Bitstamp or BTC-E, they are exchanges just like mt gox, just less volume, Bitstamp is the second largest.