Google’s fourth quarter results, announced yesterday, were fantastic by any measure – except that of starry-eyed investors and their Wall Street enablers. The company’s revenues and profits both jumped more than 80% from the year-earlier quarter. And Google seems to have continued to gain market share in its core search business from rivals like Yahoo and MSN.

But there was a dead canary in Google’s results: unexpectedly weak revenue growth in Europe, particularly the United Kingdom. Combined with the company’s heavy infrastructural investments overseas, that meant European profits accounted for a smaller-than-expected share of overall profits. Because the tax rate Google pays in Europe is much lower than in the U.S., that in turn translated into a higher-than-projected overall tax rate for the company. The higher tax rate wasn’t the cause of the earnings miss, in other words; it was itself an effect of a deeper problem.

“Problem” is probably too strong a word. Most big companies – hell, all of them (except, maybe, Exxon Mobil) – would dream of posting the kind of earnings “disappointment” that Google posted. But as Wall-Street-enabler-turned-skeptical-blogger Henry Blodgett points out, the big news here is that Google’s growth rate seems to be slowing. It’s still very strong, but it’s no longer going up. Blodgett believes that:

One likely cause of the deceleration, in my opinion, is market saturation: After five spectacular years, the company may finally be picking the last of the low-hanging search fruit (virgin queries). Eventually, growth will converge on the product of query growth and keyword price growth, which isn’t anything like 100% a year.

I would also suggest that future market-share gains will be tougher – and more expensive – to come by. Indeed, if Yahoo can get its search act together and if Microsoft succeeds in using its Windows and Explorer upgrades to push Google off a few desktops, Google’s market share may well come down a notch over the next year. So far, moreover, Google hasn’t shown any success – or even much interest – in diversifying its revenue stream.

The bottom line is that Google’s amazing growth trajectory may well have peaked. Noting that the company’s cash-flow growth slowed considerably last quarter, Blodgett says that “multiple compression [in Google’s stock valuation] is probably upon us. How much? Given the growth trajectory, 30X-40X seems more reasonable than the 50X the stock commanded a few weeks back.”

I think Google’s executives were, in their quiet way, preparing investors for such compression when they stressed, in discussing the quarter’s results, that the company would continue to spend heavily in the months ahead. In a canned quote in the second paragraph of the company’s press release announcing its earnings, CEO Eric Schmidt said, “We will continue to invest significantly as we develop innovative new products and as we extend our core technologies to new user access points and to different channels.” In a conference call with analysts, Schmidt reiterated the point, saying, according to Reuters, that “Google plans to significantly increase its capital spending this year, with most of it focussed on servers, networking equipment and data centres, as well as real estate.” “We are going to invest for the long term and make some really big bets,” Schmidt said. In addition to the huge capital outlays, Google’s labor costs are going up rapidly, as the firm continues to hire new employees aggressively. Its workforce expanded from 3,021 people at the start of 2005 to 5,680 at the end.

None of this means that Google’s stock won’t shoot up again. But with costs assured of rising rapidly and the pace of future revenue growth uncertain, you’ve got to have an awfully healthy appetite for risk to get into the game at this point. It’s a really big bet.