There were no doubt some momentary sighs of relief inside Microsoft -- at least inside the Windows division -- as the official details of Apple's iPad emerged yesterday. But should there have been?

Apple CEO Steve Jobs says the iPad will fill the niche between the smartphone and the computer. But rather than positioning the iPad as a new platform for Mac OS X programs, Apple is coming from the other direction, leveraging the booming market for iPhone and iPod touch apps. Apple says nearly all of the 140,000 apps developed for those devices will work on iPad, plus native apps made for its larger screen.

iPad: TechFlash on WHAS-AM sound bite

In that way, at least initially, the iPad doesn't seem to pose as much of a threat to Microsoft's traditional PC Windows business as it could have. But there's a lot more going on here under the surface.

Imagine, instead, if Apple allowed Mac programs from independent software developers to run on the iPad, side-by-side with iPhone apps, without restriction or approval. (Yes, I know I'm overlooking lots of technical challenges, but surely Apple's wizards could figure those things out.) In that scenario, the new device would essentially bridge the worlds of the iPhone and the Mac, giving Apple's traditional computers more momentum by encouraging more developers to write OS X applications.

But that's not how it's shaking out. Hence the probable sighs of relief in the PC Windows group.

Then again, it's a different story in the Windows Mobile group. And if the iPad becomes popular, Microsoft overall is still at risk of letting Apple define the terms of this new category, much as it did with digital music and mobile phones.

"The iPad appears to be Steve Jobs’s attempt to roll back the multi-decade trend toward more open computing platforms," writes Timothy B. Lee on his Bottom-up blog (via TechDirt). "Jobs’s vision of the future is one that revolves around a series of proprietary 'stores'— for music, movies, books, and so forth—controlled by Apple. And rather than running the applications of our choice, he wants to limit users to running Apple-approved software from the Apple app store.' "

That, then, is the reality Microsoft will be competing against if the iPad becomes a hit.

Of course, there's an argument to be made for how Apple is approaching things. As analyst Michael Gartenberg pointed out in response to my question on Twitter this morning, Mac OS X programs aren't optimized for touch screens or the iPad platform. "One reason why Tablet PC flunked," Gartenberg wrote, referring to Microsoft's failed attempts to develop the tablet market nearly a decade ago. "Lesson learned."

Also working in Apple's favor is the fact that its approval process, for all the criticism levied against it, can help to ensure the quality of the user experience and ultimately the acceptance of its own product.

Bottom line, as the industry tries to figure out this new "in between" category, we're seeing the emergence of two competing visions. Apple is applying to the iPad many of the principles made popular on mobile phones. Microsoft, with HP and other partners, is still coming from the other direction -- approaching the segment more as an extension of personal computers and the PC software model.

Which vision will prevail? And which should prevail? I'm still wrestling with the second question, but given all the buzz around the iPad, it's pretty clear that Apple has a head start, at least, on the first.