What aspects of Money Order laws will apply to merchants who use Bitcoin?
In the U.S., FinCEN just determined that decentralized virtual currencies are not like money orders.

Money orders are the mechanism enabling the moving of currency or value that serves as currency from one person or location to another person or different location.

Using bitcoins for making a purchase from a merchant is not the same as moving currency.

Now if instead of providing goods or services the merchant instead delivers currency elsewhere, then that would be transmitting money and in the U.S. State laws specify the restrictions on money transmitters.  FinCEN requires that a money transmitter register a Money Service Business (MSB) and keep records according to guidelines.