Hmmm, a lot of negative speculation in this thread... kind of saddens me.
The thing is, unadulterated search results is what made Google as big as it is today. The founders knew it, and current owners know it. That's why they keep ad-words clearly separate from results. Google is trying to provide a good product, not a profitable one. They've always expected profit to follow suit, and they report profit and astounding growth (100% annual growth in revenues).
The money is in the search results on the consumer end, but that doesn't make it right to assume they'll start selling those for that purpose alone.
You're thinking mostly in terms of their B2C (business to consumer) services, but Google is largely a B2B (business to business) provider. They make a lot of money through lucrative search deals with others (Yahoo!, AOL, etc.) and by selling their search hardware. Their B2C services result in B2B transaction profit too (adwords) but there's no evidence to suggest that they need more revenue of this type.
2 problems with this.
(1) I can't imagine anyone trying to be like Overture. They have a limited lifespan. They can provide paid results, but no-one WANTS paid results but those paying for them. Remember Disney's GO.com? They relied on paid results and failed. Who wants results from the highest bidder?
(2) The problems they have with spam? Paid results will only affect this worse. Who cares about SPAM when you're getting results that didn't earn their position through actual relevancy anyway? Secondly, among all the search engines, most would agree with me in saying Google has the most relevant results, and the least spam out of any full web search solution available (excluding ones that focus on paid results - which as I mentioned earlier should go overlooked).
General Thoughts on the Whole Issue
I don't want Google to go public simply because I don't think the trading public has the sense to own or run a business like Google. As long as the founders and current owners maintain the greatest share and only non-voting stock is sold in great amounts to the public, it's fine I guess, but I can see going IPO as a problem in the long run for the product, and then, as follows suit, a problem for the business.
In any case, more power to them, whatever their decision.