Tech Bubble Talk Boils Over

Just as worrisome, high price-to-earnings ratios are being tolerated because of the fast revenue growth that "promises" return in the form of profit growth a few years on. At the end of last week, Amazon.com's P/E was 56 -- more than three times as high as the S&P 500's ratio -- and Akamai's was an even-loftier 140. Salesforce.com was trading at 92 times 2007 earnings and more than 700 times its 2006 figure.

And in November, margin debt jumped $26.2 billion, or 11%, to $270.5 billion, the largest month of dollar increase on record and a mere 3% below the March 2000 high, according to Trim Tabs Financial Services. And while the speculation that inevitably accompanies such borrowing hasn't hit many individual stocks yet, it's possible that this time the speculative money is broadly distributed though ETF trading.

None of this offers overwhelming evidence that we are in the midst of another stock bubble -- yet. But it does serve as a reminder that the stock market is like an alcoholic: It can stay on the wagon only as long as it exercises strict discipline. When that discipline goes out the window, the bingeing will begin again.

And in some ways, the bubble debate is like group therapy: The more back-and-forth there is on the return of a bubble, the more mindful the market will be about repeating its past excesses. So let the bubble-spotters be loud and vigilant, even if they cry wolf now and then. They may be all that stands between us and another year of sober trading.