In the forseeable future, I personally think Google is going to become basically the latest Yahoo (and that's not necessarily a bad thing as Yahoo's a great site IMO and has a great business model that's currently much more well-rounded than Google's.) Heck, Google is already starting down the portal path with it's latest personalization features, email, news, customizeable home pages, etc. It still has a long way to go in terms of developing custom content that site's like Yahoo and MSN have, but it's definitely headed that way (because it's a proven path - diversity of services spreads the risk.)
I doubt they'll be charging for their primary search services anytime soon though. The ad revenue they're gaining right now is mind bongling and the costs they'd have to charge to offset that would be ridiculously high. Just from my own experience, I'm managing close to a million dollars in advertising for our various clients on Google and Yahoo this year (should be well over a million dollars next year.) And that's just from a couple handfuls of mid-sized companies. The PPC search market (and rapidly growing content-targeted ad market) that Google is the largest distributor of is only beginning to mature. Advertisers are rapidly beginning to pull funds out of their HUGE offline ad budgets (typical ad budgets for most companies goes something like 95+% offline vs less than 5% online) and beginning to pour them into online advertising. And PPC search and content ads are getting most of this windfall because the format is usually one of the most effective forms of online advertising.
That being said, at some point in the near future I do think Google is going to have to offer other advertising opportunities in addition to just text-based keyword and content ads. Advertisers and larger content sites are looking for more than just keyword ads. They want graphical ads, sponsorship opportunities and other forms of brand interaction. Google offers virtually nothing in this area currently. And if Google's not careful they're going to lose some of that advertising marketshare a few years down the road. If they're smart, they'll open their own online graphical advertising network along the lines of a Doubleclick. Yahoo's already working in that direction and actually has a major jump on them in that regard.
There's also the factor of how well Google is able to do internationally. Google is obviously wildly popular here in North America and in other parts of the globe as well. But they're nowhere near as dominant internationally as many people might assume. Regional portal sites (and again competitors like Yahoo and Microsoft) are in a much better position to challenge them in certain areas. I know Yahoo just recently bought China's equivalent to Ebay and has designs on other sites of that ilk.
Over the long haul as they evolve into other areas, it's tougher to figure out what their plans are. Their ultimate goal is all about opening and providing access to information in all its formats (not just text, but audio, video, etc.) How that integrates with with their current services and what platforms they might use (or sell) to provide that information is where the big wildcard is. In these 'new' areas, I could see them possibly considering charging for their niche search services (especially if a well-established competitor is already doing it.) But I don't think they'll be attempting to charge for their mainstream search services anytime soon.