Sending de-duplicated data to a device that de-duplicates isn't going to make much difference (in the amount of space required). its just going to change where the de-dup processing occurs (if the data domain device receives 1GB of data that it can't dedup anymore you are gonig to use 1GB data). That said, I would actually expect that the data domain device has a much better deduplication system anyhow based upon what I know of them.
The builtin backup solution isn't something I'd be using for Enterprise deployments, and if you have a data domain device, i expect you are not running an SMB shop, therefore I would continue to use 3rd party products.
The builtin backup is for smaller shops and being first release is going to have problems in larger sites.
