Wikia is getting more press the last few days as pieces of its plan are beginning to take shape.
I agree with Om though about the biggest challenge being infrastructure. You can build a great crawler, a great indexer and still not give relevant results because your infrastructure can't handle the work. Google's use of commodity hardware and in-house tools has given it a large advantage over any new startup.
The possible use of P2P technology could shift the scales a bit but until that technology becomes invisible, it won't matter.
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