The stock tumbled 10% in heavy trading, a day after the Toronto-based creator of the wireless email devices posted weaker-than-expected fiscal first-quarter numbers.

Research in Motion’s  quarterly profit jumped to $769 million, or $1.38 a share, from the year-earlier $643 million, or $1.12 a share. Revenue rose 41% from a year ago to $4.2 billion. Analysts were looking for a profit of $1.34 a share on revenue of $4.4 billion.

The company said it added 4.9 million net new accounts in the quarter, bringing its subscriber rolls to 46 million email-addled users. Research in Motion chief Jim Balsillie said RIM shipped its 100 millionth Blackberry during the quarter.

“We continue to be focused on growing our business globally and we believe that the range of exciting new BlackBerry products being released in the coming months will create significant opportunities to accelerate RIM’s growth in the second half of the fiscal year,” Balsillie said.

But the company’s shipment numbers fell below expectations, suggesting it’s losing ground in the hyper-competitive wireless market. And its simultaneous announcement that it is expanding the scope of its stock repurchase suggests RIM is hedging its bets.

By spending money to buy back its shares, the company is using capital that it could reinvest in the business to try to mollify existing shareholders — to little avail, it seems.

While mature companies in slow-growing industries routinely buy back shares to boost their profit numbers and soak up the shares issued to employees in option plans, some investors take the tactic as a sign that management has run out of ideas for expanding and improving the business.

Apple , for instance, has not been a big stock repurchaser, though some analysts have been urging it for years to return excess cash to shareholders. Google did its first big buyback only last year.

Fred Wilson, a venture capitalist who writes a widely followed blog, has seen the movie RIM is about to show and isn’t looking forward to the ending.

“RIM has been a great company that has driven so much innovation in the past fifteen years that has made my life better and the lives of many others better,” he writes. “I have to believe that if they got aggressive, they could find uses for all of that cash they are sitting on. I wish they would do that instead of buying back their stock.”