Google Public DNS is trying a few different approaches that we are sharing with the broader web community through our documentation:

•Speed: Resolver-side cache misses are one of the primary contributors to sluggish DNS responses. Clever caching techniques can help increase the speed of these responses. Google Public DNS implements prefetching: before the TTL on a record expires, we refresh the record continuously, asychronously and independently of user requests for a large number of popular domains. This allows Google Public DNS to serve many DNS requests in the round trip time it takes a packet to travel to our servers and back.

•Security: DNS is vulnerable to spoofing attacks that can poison the cache of a nameserver and can route all its users to a malicious website. Until new protocols like DNSSEC get widely adopted, resolvers need to take additional measures to keep their caches secure. Google Public DNS makes it more difficult for attackers to spoof valid responses by randomizing the case of query names and including additional data in its DNS messages.

•Validity: Google Public DNS complies with the DNS standards and gives the user the exact response his or her computer expects without performing any blocking, filtering, or redirection that may hamper a user's browsing experience.























Source:
http://googlecode.blogspot.com/2009/12/introducing-google-public-dns-new-dns.html