In a press release that came two months to the day after Tim Cook announced the retirement of Apple’s (AAPL) long-time hardware chief, Bob Mansfield, Cook let it be known Monday that Mansfield isn’t leaving the company after all.

The surprise was buried in the announcement of two unsurprising promotions — of Craig Federighi to senior vice president of Mac software engineering and Dan Riccio to senior vice president of hardware engineering (Mansfield’s old title).

Mansfield, according to the release, will work on future products, reporting directly to Cook.

What does that mean?

Nobody outside Apple seems to know — not even The Loop‘s Jim Dalrymple or Daring Fireball‘s John Gruber, two bloggers with perhaps the best sources inside Apple. The first noted laconically that the news about Mansfield was “the most interesting part” of the release, and the later pointed out that according to Apple’s revised Executive Team page, the company now has not one but two senior VPs for hardware engineering.

Mansfield’s brief retirement was, in many ways, as mysterious as his return. Our theory at the time was that after 13 years of what had to be crushingly stressful work he had decided to retire rich.

Two battlefield promotions had put him in charge of nearly everything Apple makes out of atoms rather than bits. He was part of the troika that took over Mac engineering after the messy dismissal of Tim Bucher in 2004 and was formally put in charge of the division in 2009, supervising the production of the iMac, Mac Pro and all the MacBooks. His responsibilities increased significantly two years ago when he took over production of Apple’s mobile devices (iPod, iPhone and iPad) after the Antennagate-era dismissal of Mark Papermaster.

It couldn’t have been easy to navigate the engineering space between Steve Jobs’ wild ideas and Jony Ive’s perfectionist designs. Moreover, when things go wrong at Apple, as Papermaster learned, the buck tends to stop with the senior VP.

Like many observers, we thought it was curious that when Apple admitted for the first time in memory that it had made a “mistake” — withdrawing its products from the EPEAT green registry — it was Mansfield, the only senior VP whose retirement had been announced — who signed the open letter.

We can only speculate what his return might mean. Is Cook, as some have suggested, seeking to halt a post-Jobs brain drain after the loss of retail guru Ron Johnson and a couple of iAd executives? Is he doubling down, as the press release implies, on the development of future products — a key area of concern since Jobs’ death?

Or did Mansfield discover — as many men his age have — that early retirement isn’t all that it’s cracked up to be?