Your browser has a built in page for "domain does not exist". Google cannot place ads on it. On the other hand if you set up OpenDNS and then go to "doesnotexit123411452345.com", it will likely redirect you to something like "ads.opendns.com/?search=doesnotexist", which breaks DNS, life and the Universe. OpenDNS provides an option to turn off this type of redirection, but if you have a residential connection with a dynamic IP, all of a sudden that setting might just disappear.
One of my corporate clients had their ISP move to OpenDNS for DNS services. It caused an endless stream of headaches for me as all of the various scientists employed there started complaining about the OpenDNS result pages (which also broke some other user habits they had -- they were used to being able to type "wikipedia" into Firefox's address bar and be redirected directly to wikipedia, for example).
I couldn't get the ISP to budge and the client didn't want to change ISPs, so I ended up setting everybody up on some public SBC DNS servers I know about.
With all respect due to davidu, this "functionality" in OpenDNS has caused me to feel a raging hatred for it every time its name is mentioned.
Rogers does the same thing with 404 pages here in Canada.
Another worrying thing I noticed the other day is that they have a "feature" which injects an HTML header into the page you're requesting if you are approaching your bandwidth limit.
We all know that our ISPs are inspecting our packets, but it's a little disconcerting to see the pages you request manipulated in such a wholesale fashion.
Avoid OpenDNS and IPS's that do this.