With the current configuration:

<configuration>
  <configSections>
    <sectionGroup name="system.web">
      <section name="paymentGatewayService" type="Asi.iBO.Commerce.PaymentGatewayServiceSection, Asi.iBO" allowDefinition="MachineToApplication" restartOnExternalChanges="true" />
    </sectionGroup>
  </configSections>

  <system.web>
    <paymentGatewayService defaultProvider="MockCustomPaymentGatewayProvider">
      <providers>
        <add name="MockCustomPaymentGatewayProvider" type="Crown.Imis.IntegrationTests.MockCustomPaymentGatewayProvider, Crown.Imis.IntegrationTests" authorizationAccounts="AUTHORIZE" />
      </providers>
    </paymentGatewayService>
  </system.web>
...
</configuration>

Calling myCPayment.ProcessPayment(myBillingAddress) should _always_ call ProcessPayment on a MockCustomPaymentGatewayProvider or throw an exception, correct?

What it does on my machine is call MockCustomPaymentGatewayProvider's constructor and Initialize method the first time I call CPayment's ProcessPayment method, but it does _not_ call MockCustomPaymentGatewayProvider's ProcessPayment method.  It _does_ return a successful PaymentGatewayResponse, which is scary because I don't know what gateway it could possibly be using.

Is there something I'm missing?

EDIT: Per Balaji's suggestion, I configured MockCustomPaymentGatewayProvider to be used specifically with the AUTHORIZE authorization account (probably need a new name for that, huh?); the change made is in bold. 

I also added this test:

[Test]
public void Test_ConfiguredCreditCardCashAccount_isAssociatedWith_AuthorizeDotNet_PaymentAuthorizationAccount()
{           
    var code = ConfigurationManager.AppSettings["CreditCardCashAccountCode"];
    var cashAccount = iboAdmin.ReferenceData.GetCashAccount(code);

    Assert.AreEqual("AUTHORIZE", authAccountCode);
}


which passes, showing that the configured CreditCardCashAccountCode is associated with the AUTHORIZE auth account.

The test that fails is a little much to show here, but it creates a valid credit card CPayment associated with the configured CreditCardCashAccountCode then calls ProcessPayment.  It then asserts that a MockPaymentGatewayProvider is contstructed, and that ProcessPayment is called on it.  The latter assertion fails.