Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The stock market once again followed a troubling trend Thursday, starting off the morning with sizable gains that quickly evaporated and turned into modest losses. Even though the Dow Jones Industrials (INDEX: ^DJI) only finished down 18 points, and the S&P 500 actually managed to gain ground fractionally on the day, investors are nevertheless concerned about the lack of follow-through in the bull market as it tries to extend for a sixth year. Helping hold the Dow back were AT&T (NYSE: T) and Verizon (NYSE: VZ), both of which dropped around 2%, while DuPont (NYSE: DD) was the biggest gainer in the Dow with a rise of about 1.3%.

The declines in Verizon and AT&T stemmed from the solid performance of T-Mobile US, which announced its preliminary customer results for the fourth quarter last night. Net customer additions soared more than 60% sequentially, to 1.645 million, reversing a net loss of subscribers during the previous year's fourth quarter. Those new customers came from both the postpaid and prepaid side of its business, showing that the carrier's efforts to become more competitive have been extremely successful in turning the tide. It wasn't that long ago that many analysts had written off AT&T and Verizon's main competitors as being has-beens in the mobile race; but now, T-Mobile has rewritten the story and will force the larger carriers to answer with smart strategic initiatives of their own in order to retain and build on their market share.

DuPont's gains came largely from an analyst upgrade, with Merrill Lynch giving the chemical giant a greater price target. But what's more surprising about the move is that rival Monsanto (NYSE: MON) gave back all of its post-earnings gains from Wednesday, falling nearly 3% today as another Wall Street analyst pointed to the potential for poor corn prices that could hurt Monsanto's ability to maintain pricing power over its corn seed products. With DuPont aiming at many of the same markets as Monsanto, its stock arguably should be susceptible to such events, as well, but there's no sign of that in today's price action.

Looking forward, the employment report tomorrow morning will go a long way toward determining the Dow's direction for the week. Positive numbers could restore faith in the solidity of the economic recovery, although even that might not send share prices higher if investors conclude that a withdrawal of accommodative Federal Reserve policy is more negative than a strong economy is positive.

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Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter @DanCaplinger. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.