[edit: nevermind this post. I haven't been able to express my idea clearly here.]

Suppose you have a separate block chain for domain name registrations, and you want people to pay in Bitcoins.

To register foo.domain, the buyer pays 10 BTC using the standard Bitcoin system, then goes to the other system to claim their domain name. But how to know that they paid? They can prove cryptographically that the 10 BTC payment came from them, but they can't prove that the payment was for a domain name. They might be using that same payment to claim services from several separate sites.

So obviously Bitcoin needs a way to associate a transaction ID with a Bitcoin payment. If you can include a transaction ID (of say 64 characters), then you may as well just use the Domain Name details as the transaction ID (because it's short enough), in which case there's no need for a separate domain name registration chain.

A domain registration is just a Bitcoin Payment to an unspecified miner, with a transaction ID that happens to be meaningful.