In the never ending quest to get child applications to work under umbraco do not forget to update the web.config under \Views

\Views\web.config

<system.web.webPages.razor>
<host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      <pages pageBaseType="System.Web.Mvc.WebViewPage">
          <namespaces>
              <remove namespace="Microsoft.Web.Helpers" />
              <remove namespace="umbraco" />
              <remove namespace="Examine" />
              <add namespace="System.Web.Mvc" />
              <add namespace="System.Web.Mvc.Ajax" />
              <add namespace="System.Web.Mvc.Html" />
              <add namespace="System.Web.Routing" />
          </namespaces>
      </pages>
</system.web.webPages.razor>

The other items you might need to do as well under web.config

Remove <namespaces/>

<pages>
  <namespaces>
    <remove namespace="Microsoft.Web.Helpers" />
    <remove namespace="umbraco" />
    <remove namespace="Examine" />
    <!-- namespaces -->
  </namespaces>
</pages>

Clear out <modules/>

<system.webServer>
  <validation validateIntegratedModeConfiguration="false"/>
  <modules runAllManagedModulesForAllRequests="true">
    <remove name="ScriptModule" />
    <remove name="UrlRewriteModule" />
    <remove name="umbracoRequestModule" />
    <remove name="viewstateMoverModule" />
    <remove name="umbracoBaseRequestModule" />
    <remove name="ClientDependencyModule" />
  </modules>
</system.webServer>

Clear out <httpModules/>

<httpModules>
  <remove name="UrlRewriteModule"/>
</httpModules>