Boing Boing reader Suresh Venkat says,
There's a bunch of work that computer scientists and cryptographers are doing in an area called "Private Information Retrieval" that tries to find ways of asking search queries in a way that you get the answer you want, but the database doesn't know what you asked for. I wrote a brief semi-layman's explanation of the ideas at this link.
I'm not an expert though: for more information, Bill Gasarch at the University of Maryland maintains a list of (academic) papers on this topic: Link.
And for a more practical take on this: Link.
