It was expected to happen sooner or later, and in August, it did: Notebook sales in the U.S. went into negative territory.

Sales had been growing at a brisk 30% pace in 2009 but started to tank almost exactly when Apple (AAPL) announced it would come out with a tablet computer: the iPad.

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This year, growth slowed to 11% in April, then 5% in June, then 2% in July and finally down to negative 4% in August. It's safe to say that the iPad is at least partly to blame.

Analyst Katy Huberty of Morgan Stanley calls it "tablet cannibalization," according to Fortune. And since the iPad has completely dominated the tablet market, it's not hard to connect the dots.

Sounds like September is headed for a repeat of August, with early checks showing that, at least in the first week, laptop sales were again in the negative-4% range.

If you needed any more evidence of this trend, consider the recent remarks of Brian Dunn, the chief executive of Best Buy (BBY). In an interview with The Wall Street Journal, Dunn said internal estimates showed the iPad had eaten into laptop sales by as as much as 50%.

 

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"It's a very different environment now," an electronics analyst told the Journal. "The real cool stuff now will be the tablets, e-readers and probably the higher-end digital cameras."

This week, the iPad heads to China, where response is expected to be enthusiastic. The iPad will cost about $590 there, which is firmly in the luxury category, but people still lined up at an Apple store in Beijing Friday to buy one.

Apple shares, by the way, closed at a record $276.57 Thursday, Fortune reports. The record intraday high is still $279.01. In late afternoon trading Friday, the stock was down just slightly from Thursday's close.