Although this is apparently in contradiction with what was said in bug 79672, I believe servers listed in the DNS= setting should be prepended to the list of servers. Currently they are appended and the servers coming from DHCP are given priority.
I believe they should be prepended because:
1. Locally configured settings should always be given priority over those supplied by the network
2. There is, as far as I can see, no other way of setting a preferred DNS server for all networks whilst retaining DHCP provided servers as a fallback
3. The names of the settings suggest FallbackDNS should be appended, and therefore DNS only makes sense as a separate option if it is appended
I look forward to being told I am missing the point of the setting.