Isn't it too early for that? Given the number of developers working with Microsoft's ecosystem, I don't think it will be long until they have a market as big as Apple's or Google's when it's released.
I think that WP7 wasn't a hit because MS made terrible partnerships and became a second-class OS vendor to everyone but Nokia, but I belive that the tablets will give MS more independence, will bring other makers to the table, and it will change. They're just late to the party.
I'm going out on a limb and making an educated prediction - it might well be too early. Then again, there's so much time between now and W8's release that we almost need to assume major launches and platform breakthroughs from Apple and Google (hey, Google announced Intel compatibility during the Build keynote). So to predict how things will shake out, we have to be a harsh on Windows 8 - it won't look any better next spring next to the iPad 3 and Android 4.0.
It's really weird for anyone that's followed technology for more than a decade, but Microsoft computers will have almost no special appeal to the average consumer. I don't think that Best Buy will have a special "Windows 8 Tablet" section like they do now for desktops and laptops - they'll be mixed in a sea of Android and Blackberry tablets (much like WP7 Phones at any phone store). Why not?
Windows 8 is in real danger of being second class in the consumer space. The lead development device is a Samsung, for example, but how much more profit would Sammy get if they made the same hardware with Android? Are the hurdles really great for Acer, Asus, Samsung, Sony or Lenovo (all Android vendors!) to put Android in any hardware that's running Windows 8?
I think that WP7 wasn't a hit because MS made terrible partnerships and became a second-class OS vendor to everyone but Nokia, but I belive that the tablets will give MS more independence, will bring other makers to the table, and it will change. They're just late to the party.