Seems to me like a lot of people read Brad's post and then immediately put on their Apple Fan Suit or their Apple Hatred Suit and went to town on this thread and in the process completely missed his point.
Remove Apple from the discussion. An appliance like device that is portable, very low maintenance, instant on, and can do most digital media consumption functions is a threat to the Window's portable and home computing market share. True or False?
I don't see how you can argue it's false. Many people don't use their laptops/netbooks/desktops for anything but digital media consumption. Once iPad-like devices are passed the "shiny and new" expensive phase (where many average users are suspicious of getting into the "new" thing), many people will replace their current (or maybe next) generation of laptop with something like the iPad. Why wouldn't they? If all you do is screw around on the internet, facebook, twitter, check email, and watch movies or listen to music on your laptop something like the iPad (especially after a few generations of improvements) will beat an OSX or Win7/Vista/XP hands down. If that person also has an e-reader of some sort, they can (or will be able to) replace two devices with one.
If you do more complex things (gaming, serious writing, digital content creation) you'll still have a desktop or beefy laptop.
I currently use an Asus UL80V running Win7 as my consumption device and love it precisely because it does well many of the things an iPad does well. It's battery lasts forever, its rock solid coming out of sleep, I never have hardware failures, and it has the added benefit of having switchable graphics for when I want to play some TBS style games. In my case I make trade offs (it's not as good as an iPad at the iPad's strengths, but it's still got a lot of Win7 strengths). Give it a generation or two when tablets can do more and I'll probably have one.
Though it would never be an Apple product. I despise the company and the way they treat their customers. Their business model certainly works, but I'll never buy into it.
I think Brad's point that if Microsoft wants to compete for the content consumption market they need to come up with a seriously streamlined OS of some sort is spot on and will only become more glaring as WebOS, Android, and Apple put a few generations of their devices out.