So IMHO it's not that far fetched to say that individuals and businesses are buying less PCs than they were when Win XP came along. Doesn't say they aren't buying them anymore, just less.
The only place I see people using their iPads to cash out are at the farmers market. And that makes sense since you are outside and need something portable.
But I do agree, businesses are buying less PC's but not because they are replacing them with iPads. Because they aren't spending the money. The glut in the PC market is that everyone has one. It's saturated (I'm talking the Western World here, not developing countries). Now you buy one as a upgrade. And businesses only upgrade when they must.
MS's job is to provide a compelling reason to upgrade. Time will tell if Windows 8 does this (I doubt it actually. Maybe when more apps are built for it).
the comment above you also talks about 'tablets', not 'ipads'. Tablet-style things (ie rather small devices with a touch screen) have been used for cash registers and as controls in the industry for quite some time. A lot of them were merely PC's, running some sort of Windows. This might start changing now since tablets are getting more widespread, hardware getting cheaper etc.
Which must be a pretty slow cycle in this space. I recently shopped at a store that I hadn't been to in at least a decade. I was shocked to find they were still using the exact same orange screen terminals that they were using when I was a young child.
That doesn't necessarily invalidate the original claim though. All new POS terminal installations could be using iPads or similar tablet devices.
Imagine trying to effectively generate an excel sheet. I cringe at the thought of trying to manipulate data from a tablet. You would waste a lot of time to produce something sub par. This is one example, there are many more.
Although apple reinvented the tablet market it will take greater innovation and the creation of some other tools before tablets become effective and efficient machines in the work place.