I am not sure what Dave is talking about will work either. You can't use any other Directory Service with Custom Directory. This is the only time QVS authenticates the users and it does it throught an XML file that the directory service creates. This has always been the case with Custom users all the way back to version 4.
I'll bow to your knowledge here Bill. I must admit I hardly use Custom Directory as it doesn't scale; it's only really suitable for small Proof of Concepts. I'm more likely to use HTTP Header or WebTicketing for custom authentication.
Daniel - ignoring that I might have got this one wrong generally you can only have one authentication type per Web Server because it uses a centralised config file.
Dave, I'm not sure if it was by design or by omission, but QV9 did allow you to work with AD and custom users using a single webserver, we had it set like that in a customer's test server and had to change it with QV10.