I seem to have an issue with our AD/DNS, the structure looks like:

but the BPA gives me an error:

Issue:
The Active Directory integrated DNS zone _msdcs.<domain> was not found.

Impact:
DNS queries for the Active Directory integrated zone _msdcs.<domain> might fail.

Resolution:
Restore the Active Directory integrated DNS zone _msdcs.<domain>

Now we've got 4 DC's in total: 2 running Server 2008 R2, 2 running Server 2003. The older ones will be retired sometime this year. Actually everything seems to be working ok (if something isn't then we don't know about it), we've got quite a few .NET applications authenticating against AD, no DNS issues from what I can tell and various bits on the network point to all 4 controllers.

Furthermore a dcdiag /dnsall comes up with all passes. Is this something I should be worried about?