Another question asked on Wednesday evening's Intro to R2 event was about the blob size limitation in Domain Based (ie Active Directory integrated) DFS-Namespaces. Has it changed since Windows Server 2003 SP1? The answer is no, the same limits apply. The recommendation is for around 5,000 links and the wizard in the DFS Namespace wizard reminds you of this approximate limit. However, that isn't a strict value. The real limitation is around 5MB of storage being used.
But, what is really great in Windows Server 2003 R2 is that you can get a view of the blob size very easily, and work out if you can add more links. In the DFS Namespace Management UI, examine the properties of a domain based namespace. If you have 5,000 links but the blob is around 2.5MB, you're good to keep adding links until the blob gets close to that 5MB.
Thanks to Drew from the product group who gave me the answer :)