Casino chips, like virtual property, have value only within a defined place (the casino), and are used only for a defined purpose (gambling). However, unlike virtual property, casino chips have a fixed cash value, and casinos are required by law to provide cash in exchange for their chips. (Cite to Nevada law?)
A similar law for virtual worlds would be difficult to implement. Although virtual world exchange rates are not fixed, it is not difficult to determine the exchange rate at any given time. However, the exchange rate of virtual currency is not necessarily tied to the actual revenues taken in by the virtual world provider; thus, such a law might put an online game provider at considerable economic risk if it required the provider to repurchase any in-game items before terminating an account (including shutting down the game).