It probably varies from state to state... in MA, the dealer cannot hold your deposit unless they can demonstrate "financial harm" by you backing out of the sale.
Some dealers will try to push your limits by saying that they had committed something to you, and therefore are losing money in terms of real-estate taken up, advertising costs, etc, if you back out. This may be true to a certain extent, but the law was really created to ease things for the dealer who put custom rims and a custom paint job on a car, only to have the seller change his mind. On a high demand car like the Viper, it should not be a problem, especially since they can sell to another, Viper-needing dealer without too much trouble.
If they don't return your deposit IMMEDIATELY, tell them you will be getting your lawyer involved, and going after legal fees in addition to your deposit.