That OEMs can build tablets does not mean that Microsoft can do as well. Remember, they have no such experience, and one doesn't become an expert with the very first version. Second, Microsoft's competition is certainly not those traditional PC makers, but Apple and Google. If the "competitive" price for the Windows RT version (ARM) isn't the same as the iPad or less, Microsoft will be in trouble. For the Windows 8 Pro version (Intel), they'd have to be priced the same or less than mainstream Ultrabooks or the MacBook Air.
But the larger obstacle to Microsoft is getting it right the first time, and having enough Windows RT apps to seem competitive to the iOS. You don't build a large app ecosystem overnight.
There are lots of unanswered questions, but the biggest one is why the media couldn't get extended face time with working prototypes. If the ARM Surface is to be out this fall, when Windows 8 is set to be released, you'd have to expect that the prototypes would be very near final. That doesn't appear the case, since Microsoft would have no reason to prevent the press from actually playing with the things. Certainly when Apple makes units available at a media event, they work.
Right now, the 2012 Surface is vapor. If Microsoft can meet the deadline, and get working units that pass muster by reviewers and bloggers, you'd have to take them at least somewhat seriously. We still don't know.
And don't forget that Microsoft is notorious for showing off product prototypes that never see the light of day, or show up severely feature-limited. Even Windows Vista was less than its advanced publicity.
