Ford CEO Alan Mulally at the New York debut of the new 2015 Mustang.

Ford CEO Alan Mulally appears to be sending some very mixed signals when it comes to the ongoing rumors about his candidacy to become the next chief executive officer at Seattle-based Microsoft – but the software giant’s stock slumped when Mulally indicated there will be “no change” to his previously announced plan to stay on with Ford until at least the end of next year.

The former Boeing executive has been the center of rumors swirling around Microsoft’s need to find a successor to Steve Ballmer, the controversial current CEO who has said he plans to depart by next August.  While the Microsoft board of directors has acknowledged only that it is looking at a number of candidates, there have been numerous reports that Mulally is the tech firm’s top choice.

The timing might almost work.  The 68-year-old Mulally has stepped back in terms of handing day-to-day duties over to Mark Fields, a year ago named Ford’s chief operating officer and considered the front-runner to be the automaker’s next chief executive.  After seven years in Detroit, Mulally has publicly stated he plans to stay with Ford until the end of next year, and in an interview on CNBC, he suggested there has been “no change in my plan.”

That send Microsoft stock tumbling by 3%, or $1.09 a share, to $37.85 at Thursday’s close. The stock has been riding a roller-coaster in recent weeks as investors try to get a clearer sense of the succession process that will replace Ballmer who himself had succeeded MS founder Bill Gates.  Ballmer’s announcement earlier this year gave the Seattle firm’s stock a big bump.

(Click Here for a first look at the 2015 Ford Mustang.)

Numerous observers have suggested that Mulally would be perfect for Microsoft.  A senior automotive official who previously worked at Microsoft – and who asked not to be identified by name – said the executive would have “much the same challenge” if he were to return to Seattle.

Mulally has been seen as a unifying force at Ford, helping tie together traditionally independent and often conflictive units of the automaker’s global empire.  That, the source stressed, is much what Microsoft faces.  “They have tremendous talent there,” he noted, “but the various parts of the company need to start working together” more effectively in order to challenge rivals like Apple and Google.

(New Mustang was a difficult challenge – and the last big project – for retiring Ford design chief J Mays. Click Here for the story.)

While Mulally’s suggestion that he would stay on through the end of 2014 at Ford might disappoint some observers, he pointedly would not dismiss the possibility of ultimately going to Microsoft.

The executive brushed off questions from CNBC’s Phil LeBeau by declaring, “We don’t comment on speculation on any of the rumors that are out there about any subject.”

Even before the Microsoft rumors had begun to circulate, the question of how much longer Mulally would stay in Detroit had already become a hot topic.  With the 52-year-old Fields now handling day-to-day duties, it had been seen as increasingly likely Mulally wouldn’t stay on past 2015 – though he has said on a number of occasions he didn’t see the need to depart as long as he continued “having fun” in the job.

Considering there’s a relatively small gap between Ballmer’s planned August 2014 departure and the time Mulally has committed to staying in Detroit, it could be possible Microsoft might simply have to delay its succession until the beginning of 2015 – if, that is, the MS board makes a pitch to the Ford chief executive that he would accept.

A critical question is whether Microsoft will, in the end, want to bring on a top exec who would, by then, be approaching 70.  In years past, that would have seemed highly unlikely as most top managers tended to retire by 65. But there’s been a swing towards keeping top talent around even after traditional retirement age.  Former General Motors Vice Chairman Bob Lutz, for example, was approaching 80 when he left that company. GM Chairman and CEO Dan Akerson himself turned 65 in October and is so far unwilling to set his own departure date.