Question : new domain tree - new AD site ?

I’ve just created a new 2008 r2 domain tree in my 2003 forest. All my users will be migrated to this new domain, and the current domain they reside in will become the parent/schema domain. Im going to be using ADMT to migrate the users accounts.
The new domain DC’s are currently in the same AD site as the parent domain DC’s. Should I create a new AD site for this New Domain and assign a new IP scheme to the migrated users?
Im concerned that if both DC (parent domain and new domain tree) are in the same site users will still try to authenticate to the parent DC and might be denied if the parent DC does not have the users AD account anymore, or will the parent DC do a lookup on the new domain DC’s?  
Could there be other foreseeable  problems with objects getting migrated to the new domain and authentication requests still using the parent DC’s (in the same AD site) for info that no longer exists in the parent domain?

Answer : new domain tree - new AD site ?

You are going to have to use OWA on one listener with FBA.  I don't care if the FBA is from the OWA box or use the one from ISA,...but it is best to do it with the ISA and let it delegate to the OWA box.

Then the RADIUS can only be used with the other two sites using a different listener.   I don't see anyway to have RADIUS with all three sites on one listener unless you went totally without FBA