My first reaction to the headline was, "Oh no, not again..." (having already weathered both the OS9 -> OSX and PowerPC -> Intel x86 transitions)...
But after that initial groan, a few other (more positive?) considerations came to mind.
First, Apple really did do a great job of transitioning from PPC to Ix86... it was far less painful than it could have been. Not perfect, but incredibly well-managed.
Now, OSX Lion is coming, and it appears to contain the beginnings of a convergence and consolidation between iOS and OSX. If we try to imagine where those OS's will be, say, 3 years out (and the hardware as well), by THAT time, it may be as simple as flipping a switch and hey-presto, you're on an ARM device without missing a beat...
I say this because, as devices like iPad evolve over the next few years, the applications written for them will also, and by the time 'higher end devices' like desktops and laptops are lining up for a platform change, those "mobile" app versions will already be 'full featured', and already written for ARM-based devices (I'll use the current Garageband pair - with cross-compatible OSX/iOS versions - as a very early-market example of that future). So, the painful prospect of rewriting/recompiling all your code won't be nearly as bad as it was for the OS9->X transition.
Another consideration is that tomorrow's mobile devices will be far more powerful than even today's desktop/laptops are. It's harder to imagine the future of the desktop/laptop as we know them today.
In fact, now would probably be a good time to remember that what Jobs is creating here isn't just "magical devices"... he's embarked on defining the "Post PC Era"...
It'll be interesting to see where all this leads, but my take on it is that it might not even feel much like a "platform switch" by the time we arrive there...