I think you"ve somewhat missed the critical difference between "Purpose" and "Possible uses". The purpose of the Raspberry Pi is quite clearly stated, viz:
… an ultra-low-cost computer, for use in teaching computer programming to children.
The fact that it could (and most probably will) be used for other purposes is quite beside the point. That goal, and that goal only, is the reason the Raspberry Pi foundation exists, and the reason the Raspberry Pi will hit the streets.
Raspberry Pi Purpose : teaching computer programming to children
I personally see Arduino as being orthogonal to the Pi; it exists to take the "programming" out of hardware experimentation. Learn about electronics and microcontrollers without having to learn too much about all that fussy "coding" stuff (and certainly without having to learn about interrupt coding and assembler and embedded controller event loops). To use Arduino as a base for teaching programming would largely involve throwing away the Arduino toolset and getting people into developing using AVR assembler or embedded C/C++. Not that this would be a bad idea, mind, but at that point you have a wealth of choice (the various AVR boards - Teensy etc, Propellor, the wealth of venerable 8x51 solutions, ARM-M3, and so on). In any case, it requires a tether to the desktop for programming.
Conversely, to use RasPi for what Arduino does, you"d need not only to redesign the board to incorporate the various physical protections required and to make the IOs physically accessible, but you"d also need to hide all the "programming" stuff. While the latter part might well be feasible (and could probably be done "on board", from what the foundation people have said here, the former isn"t gonna happen.
The tablet market exists to make money selling gadgets to people who have more money than sense. Especially at the lower end, where the feature set is limited and performance is, at best, "poor". Android itself is a poor tool for teaching programming (it"s hard to program on the device) and a worse one for electronics experimentation. It"s also hampered by the fact the underlying hardware is usually totally undocumented, and getting source for the kernel used (usually required in order to boot something more suitable for programming) is like pulling teeth. Your own teeth.
One could use Android as a teaching platform for programming, but hundreds of dollars per unit plus desktop machine for programming makes the cost orders of magnitude more than the Pi.
This is a very productive discussion as it may provide an impetus resulting is an evolved purpose for the most worthy Pi mission. Children may gravitate to this project in droves after seeing what early adopters have achieved as a result of exploring the new bleeding edge afforded by the implied expansion of the original purpose.
Please don't be frustrated by the appearance of augmentative discussion but be encouraged by the possible vistas that may open up. The limits of a child's imagination is only constrained by superimposed boundaries.
Well Done All