Uhm... he never perpetuated the idea of the 3rd device. Otherwise he never would have said "PCs will become trucks" or coined the term "post-PC era." He knew there was a market for those that didn't need the complete PC functionality but wanted something portable to browse the web, email, play games, etc. on. And it's true - a lot of laptops were sold not because people needed the functionality, but because they already had a desktop PC but also wanted portability.
What everyone on this forum fails (and has since the iPad was introduced) is that ordinary people hate PCs (even Macs). They can't stand them, they fail to understand why they are so difficult to use when all they want to do is look at photos, send email, and browse the web. The iPad stepped in and said "Hey, you can do all the things you want to do with a PC, but without the PC part. Here's a device that's easy and can meet your needs." The market responded. That's why tablet PCs have never taken off. That's why Apple was smart not to put MacOS X on the iPad. Most people here have never understood that.
So how does this Surface fit in? Well, I don't think the Intel version will be very successful for the reason above, and the ARM version will depend on the apps available when it ships.
For a post PC era Apple sure is putting a lot of effort in making and selling PC's. No, the ipad was meant as a device in addition to your PC, not as a replacement. Sure I get that people like simple devices, but Microsoft is putting a TON of effort into simplifying Windows 8. You can use Windows 8 in the metro style and never leave metro, and it functions very much like a simplified OS such as iOS. Or you can go into the old desktop mode and have a full Windows machine, the consumer can decide.
The reason why tablet PCs didnt take off are because of things like battery life, size, UI, OS not being suited for a portable touchscreen, etc. Things which Apple elegantly solved and deserve a heck of a lot of credit for. Microsoft realizes this, although very late as is their way, and that's why they've completely reinvented Windows.
I think if people hated windows so much they wouldn't sell so many laptop licenses, or Apple wouldn't sell as much OSx as it does, or we would have iOS on all of our desktops and laptops. Obviously this isn't the case, so someone is buying the Windows OS, and it's not just corporate sales. That's the beauty of the surface, it truly IS a "post PC era". You can truly buy one and say goodbye to your laptop. Microsoft providing Windows licenses and the only hardware they make is the post PC tablet is great for them if we have a paradigm shift away from laptops and home PC's, something which would be terrible for Apple.
In terms of which will sell more, I think the opposite, I think the Pro model will be insanely successful, especially if a lot of 3rd parties pick it up like HP, Dell, etc. I think the RT model will fall flat on its face as people realize they don't want a watered down OS, it's only saving grace will be price and possibly battery life.
