People didn't think there was a market for e-readers either, but the Kindle is actually catching on. These would be a great replacement for it, just port the books to PDF and add some DRM to keep Amazon happy.
To parrot my thoughts from the other iPad thread, I did not think the Kindle would do well.  It's actually a fine product and, although I dont own one, I have purchased three as gifts for others, who love them.
Therein lies the difference, however.  In my mind, the Kindle is focused.  It has a specific purpose and it excels at that purpose.
By contrast, what does the iPad/G-Tablet represent?  I cant figure it out.  Its not a fully fledged computer, and its too ungainly to be used in a truly portable fashion like a smartphone.  The iPod and the iPhone were both revolutionary for their respective industries; they are each genuinely useful and are both better than what they replaced.  The iPod replaced CD's and the iPhone replaced call-only mobile devices.
In what industry/niche are these tablets competing?  What are they revolutionizing?  To whom does Apple and Google expect to sell these things?  
If you're a foreman; are you going to be bringing one of these to a work site instead of a "heavy-duty" laptop?  Hell no.  Drop it once and the screen will probably shatter.  
CFO going to be using it?  Nope.  He needs a proper computer to run proper software.  
Point is - the tools for each job these tablets can perform already exist - and are each respectively better at those jobs.
How long has the computer industry been trying to push using your computer as a television?  15 years now?  Ain't gonna happen.  Yet I still see "HDMI-Port!  1080p Capable!" as a feature on today's video cards.  Why do I care?  
With 3DTV being the next "big thing", you can buy a friggen 65" 1080p DLP HDTV for a grand these days.  Who is going to spend twice that on a rig and a 25" monitor; and watch TV at a desk; when you can skip that option and spend the remainder on a nice comfortable couch from which to watch your brand new, obscenely large television?
I see a lot of these tablets ending up on school buses and in coffee shops; but only for those who want to be seen with one... on a school bus or in a coffee shop.
DOA.