Wow Marcus, you completely misunderstand products on their effect on other products a company sells.
For example, it's well known the iPod has helped sell many more Macs, simply because it creates more brand loyalty, and brand recognition, not to mention gets them into the Apple stores.
The iPad will help more, not less. The interface is, obviously, much more similar to a Mac than Windows, and therefore makes a Mac a much better companion. The implied supposition that only Apple people are buying it anyway is completely flawed. Apple constantly creates new markets, and gets new customers with it. Some will be people with Macs (who like the interface for the same reason a iPad owner will prefer the Mac, similarity), but some are all new, and now they have a relationship with Apple, and when they need a computer (few if any will stop buying a computer because they have an iPad), Apple will be on the short list.
That's why Apple's earnings are so spectacular. Success in one area begets success in another. It does not cannibalize it.
The iPad will help Mac sales, not hurt them. It already has. So has the iPod, so has the iPhone.