Writing for MacWorld, John Gruber makes a case for the near-term persistence of the Mac, but casts doubt on its long-term relevance, and linking to the article from his own blog, the link text he chooses is "All Good Things Must Come to an End" and the article provides an implied subtext 'just not right now.'
Why so much gloom over the Mac's future when sales are through the roof? As Gruber says himself "The irony is that there’s more doubt today about the long-term prospects of the Mac than there has been at any time since Steve Jobs returned to Apple in 1997." The driving factor isn't Windows, of course, but iOS, Mac OS X's younger, more svelte sibling. People look at sales figures for the iPad, note it is already selling faster than the Mac, and start counting the days, weeks, months, or years until the Mac's demise.
Time for a reality check!
To begin with, iOS and Mac OS X are far more similar than different. The main difference between them is in the libraries supporting the user interface, AppKit on the Mac and UIKit on iOS devices. Below that level, they're practically identical, and becoming more nearly so with each release. (Parts which were originally left out of iOS due to resource limitations can be folded back in as more capable hardware becomes the norm, and some parts which originated in iOS are finding their way into Mac OS X.) The truth is, following their initial divergence to enable support of a touchscreen interface and limited hardware, and the fact they remain on separate tracks for the time being, in the long term they will probably reconverge. I'll come back to this point.
Not very long ago, "Mac" meant a machine with a keyboard, a pointing device (mouse or trackpad), an optical drive, a few ports, and a screen at least 13" from corner to corner, with a dual-core (or larger) Intel processor combined with a multi-core dedicated graphics processor, and running Mac OS X. The MacBook Air removed the optical drive from this definition, and more recently dropped the lower limit of the screen size to 11".
On the other hand, whereas the original iPhone required a bit of hacking before you could use an external keyboard with it, the iPad had a keyboard dock available in roughly the same time frame as its own release. Obviously, Apple recognizes an on-screen keyboard isn't an acceptable substitute for a physical keyboard for many purposes. And if they haven't yet made it possible to use a mouse or trackpad with an iPad, they certainly could. If they're holding off, it's probably because they're working on a comprehensive solution for the combination of two subtly different user interaction paradigms.
This is how I see iOS and Mac OS X converging, through each gaining the ability to support the other's UI paradigm. Just as you now see keyboards connected to iPads, you might also see touchscreens connected to Macs, something which has actually been possible for awhile, thanks to Wacom, and I believe there are also apps which enable the connection of an iPad as a touchscreen peripheral, although they probably pair with specific Mac apps, rather than providing general touchscreen utility.
In very simple terms, this means building a version of iOS including AppKit, and a version of Mac OS X including UIKit. The reality is no doubt a good deal more complicated, but that's the nutshell version.
For the developer, the path to taking maximum advantage of this is through distributing app components across the whole range of devices, with each device running the components that make sense for it, given its intrinsic capabilities. For Apple this presents a choice between leaving developers to work this out for themselves in a hundred different ways or to provide a framework which makes it straightforward. I can't imagine Apple wouldn't choose the latter.
So, if the Mac disappears at all, it will be disappearing into something larger and even more powerful, pieces of which will fit in your pocket, or on your wrist, but most likely there will continue to be machines called Macs, using a keyboard and pointing device as their default paradigm, until the market for such machines shrivels up, by which time most of us will have ceased to care, else there would still be a market.