I ran into this issue today when finally rebuilding my lab DC01.Contoso.com and using DCPromo. This warning pops up and since I had crippling DNS issues in the past (Thanks again Mike D!), I got really nervous.

Solution: It seems that this message can be safely ignored in my case because according to the article that I finally dug up (http://social.technet.microsoft.com/Forums/en/winserverDS/thread/38ccac86-375c-4402-9bfc-660ce5c0a606), I only have a single domain in my lab environment. I also confirmed DNS health with the “dcdiag /test:DNS” command. All’s well.