Same with MtGox. We deposit fiat, and we get IOUs which are a database entry in their MySQL server. Then you buy Bitcoins on the exchange but you are really getting Bitcoin IOUs which also sit in their database. When you withdraw US Dollars from MtGox you get IOUs from your bank instead of MtGox IOUs. It is only when you withdraw Bitcoins to a wallet under your control that you actually have something which is not debt based and without counterparty risk.
What Ripple nay-sayers don't realize is that Ripple functions identically to the MtGox scenario, it's just more explicit. It would be like being able to send your "MtGox US Dollar IOUs" in a cryptographically secure way to anyone else with a Ripple wallet. Clearly, this is useful and not a "scam" as forum trolls would have us think.