The latest Hitwise stats show that Google has broken the 60% mark in search engine market share. Hitwise data has Google at 60.2%, Yahoo at 22.5% and MSN at 11.8%. Gigaom.com asks, in relation to that:

"…this begs the question: When it comes to search can Google get as big as Microsoft is in the desktop?"

I think there are key differences in the search market compared to the desktop one, but it is a fair question Google's dominance in search won't ever reach monopolistic levels when you consider that the Web is the main development platform nowadays - usurping the desktop. So on those terms you could say that Microsoft's dominance has also been usurped, by Google. 

Also there is a lot of leverage to be had in being the dominant search engine - particularly in online advertising, but also in terms of promoting its other Internet properties (i.e. being a type of portal). So far Google has executed extremely well on the former (advertising), not so well on the latter (portal). In that respect I question whether Google can ever get to the 90%+ mark as the dominant search engine. As more and more people use Windows Live services (still a big 'if', I'll grant you), then Microsoft should see some of that translate to higher search engine market share. That is part of their Windows Live strategy - to convert web app users over to the new Windows Live search engine.

Also, I think the rise of search 2.0 engines (aka social search) will probably affect Google's market share in the search engine space - not for a few years probably and I don't think Google's position as number 1 will be threatened. But I do think the popularity of search solutions such as Clusty, Lexxe, Qube will have an impact, over time, on Google's market share.

So I have to say that no, Google's dominance in search won't ever reach the 'monopolistic' levels of dominance that Microsoft has historically enjoyed on the desktop. That's probably a good thing, because search is definitely a market that thrives on innovation. There's a lot more improvement to be made yet in search, so having a 90%+ dominant search engine would throttle that innovation.