Another example: locate a smooth flat surface. Now drop two grains of salt on it. Can your fingertip feel the difference in height?
If human hands and eyes were so poor in seeing small differences, there would be no brain surgeons on this planet.
While I agree with you on the ability to detect small differences; the real issue is will such a difference be noticed over time? In general, the answer is no - people become used to the new dimensions and they then feel normal. So, unless you use an iPad2 and the (mythical slightly thicker) iPad3 interchangeably and swap often enough to retain the memory of the iPad2's thickness you will not likely notice the difference.
For me, I'd like a retina display iPad that could act as a portable 2cd monitor for my MBP so I could display a whole A4 or 8x10 sheet of paper on it and edit documents a page at a time.
Well, they will need to provide much better connectivity and a file management system if they ever want to be taken seriously as a computer replacement.
While that would be nice, I'm not sure it is necessary as long as there is good file interoperability. Most people who I have worked with, over various industries, use their laptop to:
1) Check email (generally via Outlook)
2) Write using Word
3) Put together Presentations (Powerpoint)
4) Use Excel
An IPad can already do that, albeit without MS native aps. If the Office suite was available on an iPad, then an iPad would be a viable light weight, cost effective alternative. To me, the real limitation is storage memory; especially if you wind up storing an entire mail file, with attachments, locally. My work files backup, which includes a .pst file and all my work documents is pushing 30+GBs, so a low end iPad would not have the storage for me to replace my MBP. I could move to a different storage model, such as cloud/server/iPad but that negates the "I have it here, anywhere" factor.
Still, an iPad with 64GBs and Office would be a great "through in the briefcase and leave the MBP at home" device. If it had the ability to attach to a projector and do presentations like on the MBP w/Powerpoint it wold be even better. I'd probably toss a keyboard in my bag, for longer editing sessions, and would love Apple to add a mouse driver that mimics the touchscreen functionality.
While I am dreaming, why not add a virtual laser pointe - touch a location on the iPad display and the projected display highlights the chosen section. Write on the projected image by writing on the iPad display. Sort of an electronic white board.
Damn, I want one now.