Update: David Findley posted something that I hadn't thought of using that's even easier.  Adding this to web.config will dump email messages sent from an ASP.NET application to the specified path:

<system.net>
  <mailSettings>
    <!--
    Production setting
    
    <smtp deliveryMethod="Network">
      <network host="localhost" port="25" />
    </smtp>
    
    -->

    <smtp deliveryMethod="SpecifiedPickupDirectory">
      <specifiedPickupDirectory pickupDirectoryLocation="C:\TestMessages" />
    </smtp>

  </mailSettings>
</system.net>