Banks (in the US) don't loan out deposits; deposits are the cheapest source of reserves used for settlement, but the actual loans themselves are created ex nihilo [1].
This is kind of the central point of why we no longer experience bank runs in the broader economy, and why Bitcoin will always be vulnerable to them.
Does anyone know whether Gox keeps 100% of deposits in reserve?
In a somewhat similar situation-- online poker, where poker sites took deposits in a wildly unregulated space-- Full Tilt Poker only kept a small amount of deposits on hand and paid out much of it to insiders.
This is kind of the central point of why we no longer experience bank runs in the broader economy, and why Bitcoin will always be vulnerable to them.
[1] http://www.standardandpoors.com/spf/upload/Ratings_US/Repeat...