Interesting question in the face of the DNSChanger virus. Infected machines will not be able to access DNS if the FBI disables the pirate servers on 9 July 2012.
But to your question, that is why the DNS system is presently being overhauled to include top-down validation. In addition, there are certificate-based HTTPS websites and other methods of URL validation (at least of how safe they are expected to be).
In short, this is a five year old question that is answered by keeping your protections (firewall, antivirus, OS, and application software) updated with current patches and not trolling websites that are not certificate protected. NO PORN.
Can we close out this question now?