Hi Joe,
I'm not sure that this is possibly, but what you could do is use the AD Web Service that we built. We do this all the time where the user types in the alias and then all of the other information (such as DisplayName and e-mail address) is pulled down from AD.
This doesn't quite give you a drop-down, and the users must know the alias of the person they're looking for, but it's all I can think of. Hope this helps!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge