Search is an unsolved problem. We have a good 90 to 95% of the solution, but there is a lot to go in the remaining 10%.
I agree with Michael that search isn’t even close to being solved yet. I’ve criticized the way many web search start-ups–and even the giants Yahoo and Microsoft–are going about trying to dethrone Google through incremental improvements or technologies that don’t address any need that Google does not already adequately (if not optimally) address. But there is no lack of open problems in search for those ambitious enough to tackle them.