WARNING: when you make a backup of com.apple.mDNSResponder.plist, DON'T leave it in /System/Library/LaunchDaemons!! I tried making this change and had copied it to com.apple.mDNSResponder.plist.backup in the same directory and totally broke name resolution. Tried backing out the change by copying the backup file back over and still broken. Static IP with DNS addresses set manually; still broken.
What gave it away was that I could get name resolution to work again with
but resolution would still be failing after a reboot.
The admonition in the article to delete a ....plist~ file if you were using Emacs gave me the idea that maybe this was one of those things where anything in the directory gets used somehow regardless of how it's named and that made me wonder if the presence of com.apple.mDNSResponder.plist.backup was causing a problem. Deleted com.apple.mDNSResponder.plist.backup and all was well again. I was looking at a nuke-n'-pave!