I'd have to step through the code to see if Domain Users even comes up as a property that ADSI can see. I'm just checking the AD here at work (where I don't have full admin permissions) and the scope and type for Domain Users is grayed out. I'll have to try and check my home network to see if it's the same. If it is then I'd guess that because it's a required (by Microsoft) group that it may not get polled when the ADSI code (which is what the AD provider uses to talk to the AD) looks for the user's groups.

That said.... You say that you've got the module setup for Unauthenticated users and the other setup for Domain Users. Are you external users allowed to login? I.E. They have DNN accounts? If not then why not just set the permissions to Authenticated Users?

Hello, 
Thanks for the reply.. Yes, we're creating our intranet site as a child portal from our main live site.  Currently, we're still on the development side of things for testing, etc.  Kind of a, can it be done type of scenario right now.
There are internal forms, etc which we'd like people externally to access whenever they wish.  If they're at work and on the network, they'd hit the same portal and be auto-logged into the intranet site and away they go.  So yes, if they're external, they'll log into the intranet site using the network credentials.  Both of these options work perfectly for us in testing; well, externally isn't really being tested, but Opera requests username/password in order to see the site and it'll log in using network credentials.
Hmmm, I wonder why it doesn't see the "Domain User" group.  I figured I'd try that as a DNN security group since every user created on the network is automatically assigned to that AD group anyway.
One way I got around this and got it to work was assigning the auto-assign checkbox to the Domain Users security group.  This way,  when the user logs in, their credentials are checked with AD and if it's good, then they're auto-assigned to the Domain User security group and they can see everything on the page.  Probably not the best security configuration, but it does seem to work.  But since these users are only being assigned vew permissions to the page and modules, I don't think there'll be too many issues.
Thanks.. -Jeff