Here's an old DNS proposal using Bitcoin's general timestamping properties:
http://privwiki.dreamhosters.com/wiki/Bitcoin_DNS_System_Proposal
(This could be greatly improved.)
Ah, I think I now remember hearing these objections before regarding Namecoin. In my opinion the key would be to have a generic system, even more than worrying about the block chain baggage - you really don't want to start polluting the code base with for example NC specific code.
Incidentally this is what I probably like the most about Bitcoin - it exposed so much uncharted territory