DDG is flattered by GOOG's imitation of Lockable Safe Search
duckduckgo.com
duckduckgo.com
So how does the DDG solution actually solve the problem?
I wouldn't want to be blocked based on IP, especially since it appears your implementation doesn't allow it to be undone. People search for porn, including parents. I like what DDG is up to, but this implementation is majorly flawed.
This is a cool idea, but, as you admit, exceedingly error prone. There is no reliable way to determine who actually owns an IP address. With dynamic IPs it can even lead to one user blocking search for another user. Lock safe search, renew DHCP license, repeat.
How do you plan to deal with these conflicts?
Otherwise, it's not all that hard for one Comcast or AT&T user to lock safe search for their neighbors.
It's obvious that you don't care right now, but your current algorithm clearly creates a diminishing market for "unsafe" search users, until they're forced finally to buy themselves fixed IPs which hopefully haven't been locked by the previous owner.
I'll stick my neck out here and say that, dammit, I ought to have the ability to find pictures of naked people, and that someone else should not be able to make that choice for me. And, furthermore, that there's nothing wrong with that.
One weird thing I've noticed: your normal search is more informational and less commercial than Google's. For example:
http://duckduckgo.com/?q=credit+cards&v=
Dmoz and the FTC outrank "creditcards.com"!
Are you disregarding a lot of the stuff companies do for SEO? Or what?
We also have a non-commercial search type accessible from our home page that further tries to reduce commercial links.
I suggest "Splooploo".