From the Diablo 3 AH FAQ

Q: Can players choose to get cash from currency-based auction house sales, instead of having the proceeds deposited into their Battle.net account?

A: Yes, as an advanced feature, players will have the option of attaching an account with an approved third-party payment service to their Battle.net account. Once this has been completed, proceeds from the sale of items in the currency-based auction house can be deposited into their third-party payment service account. “Cashing out” would then be handled through the third-party payment service. Note that this process will be subject to applicable fees charged by Blizzard and the third-party payment service. Also, any proceeds from the sale of items in the currency-based auction house that have been deposited into the Battle.net account will not be transferrable to the third-party payment service account. Not all regions will support this advanced feature at launch. Region-specific details, as well as details regarding which third-party payment services will be supported and the fee that Blizzard will charge for the cash-out process, will all be provided at a later date.