More on this problem.  While using the AD Authentication in the main section works fine, when a user enters a portal he loses his login.  When I go to admin|authentication to try to use the AD Authentication in the portal so that the user should be able to seamlessly pass from portal to portal - I get the following error:

Error: Windows Authentication is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Security.Authentication.ADSI.Utilities.AddADSIPath(String Path, Path ADSIPath) at DotNetNuke.Security.Authentication.ADSI.Utilities.GetRootEntry(Path ADSIPath) at DotNetNuke.Security.Authentication.ADSI.Utilities.GetRootEntry() at DotNetNuke.Security.Authentication.ADSIProvider.GetNetworkStatus() at DotNetNuke.Security.Authentication.AuthenticationController.NetworkStatus() at DotNetNuke.Modules.Authentication.AuthenticationSettings.cmdAuthenticationUpdate_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

Any help on this would be greatly appreciated

What settings are you using on the authentication for each portal? 

I have this setup on the main and child portals, and am able to go back and forth without any issues.  Basically, the settings must be the same for each portal's auth. (from my experience), and I'm using delegation on both.  You will have to login once on each portal as it is set up with AD - it has to synch the first time, as each will be a user on each portal they choose to access.  From there, it works fine loggin in and out from each direction. 

You should make sure to create all Roles matched with the AD groups you seek to synch before allowing users to log in. 

My only "issue" is that we have to put in the domain\ and we have several domains to manage.  We are starting to test conflicts in the AD groups/Security Roles assigments to make sure these are not polluted inadvertently.