Hi,
I think you might find that using our Active Directory Web Service (ADWS) is an easy way to do this and will give you much more informaition than you can get using the contact selector in InfoPath 2007. The contact selector in InfoPath 2007 only returns the logged in users's alias and name from the SharePoint user list. If the user is not in SharePoint yet (hasn't gone to the site) then their alias may not show up (this is an issue for example if you need to assign the form to someone who hasn't logged into the SharePoint site ever but is still a user on your Domain).
ADWS allows you full access to your GAL in your forms for capturing both the currently logged in user and also allows you to search for users in your domain for workflow purposes.
Information about ADWS is here: http://www.qdabra.com/en/products/ADWebService.aspx
Ted