Google has made public DNS servers available for one simple reason: They want google.com to resolve using their IPs (74.125.93.xx) instead of other IPs. Notably, if you use OpenDNS, a request for google.com resolves to OpenDNS’ 208.69.36.xx IPs.
This takes, or has the potential to take, ad revenue from Google, so Google responded by having very easy-to-configure (how hard is it to forget “184.108.40.206” and “220.127.116.11”?) DNS servers that don’t include any ads—besides the ads Google themselves include in their search result.
They made the DNS servers geek-friendly: Their DNS servers do not redirect NXDOMANs (DNS replies saying “this host name does not exist”) to an ad-filled page, but correctly forward NXDOMAIN replies on to the end-user.
There’s a good reason OpenDNS’ president is scared of Google’s DNS servers; by offering for free (ad-free) what OpenDNS offers, they have undercut OpenDNS and, quite frankly, OpenDNS’ days are numbered. Yes, they do offer spam/phish/adult content filtering, but the market for that is smaller than the market for “My ISP’s DNS servers suck, give me something more reliable”.
Disclaimer: I sent my resume to OpenDNS a few months ago and never heard from them; while I didn’t get hired at Google, I had a very pleasant experience interviewing with them a few years ago.