This has been discussed, but not tried to my knowledge. The trick would be to extend the web application one additonal time. When you configure authentication, do the same thing you did to configure the app to use ADAM except this time point it towards your Active Directory. Of course you'll have to have network connectivity between the SharePoint box and an internal  DC to do this.

The other option is to use the ADAM proxy bind feature which reroutes authentication for a particular class of ADAM user objects to Active Directory.