Now to answer those questions:

1)
It will be your home entertainment unit.
It will spend its idle time on the coffee table infront of the television in the living room.
When not used it will be an electronic picture frame
It will be:
your Family organizer/schedule
the thing you write notes in for your family (Honey food in oven etc)
for web browsing from your couch including flash
your Music/Movie player (smooth 720p flash)
your Networking device (video chat, facebook etc)
your sketchpad, notepad and Document editor
your console to play flash games and emulators
Connects to your HDTV

2)
Need:
16:9 720p screen
Touchscreen
Wifi
Front Camera (3MP)
HDMI
SD card
Loudspeakers
Microphone

Expected:
Bluetooth
Accelerometre/Gyro
infrared
internal storage
Video capture

Want:
Capacitative Multitouch
C-tylus with dedicted slot
Wireless HDMI
GPS

3)
I think it should be a 10" touchscreen.
The screen would be covered in 0.5" bezel on the horizontal ends and 1.5" bezel on the vertical ends.
The bezel is black/white, continuous (borderless) and covered in glass ... eg iPhone
The horizontal bezel has no features.
Two loudspeakers are centred on the vertical ends.
The vertical bezel should include a camera (on the top right corner), and two customizable buttons oppositely arranged (on the bottom corners), and a home button (on the top left corner).
On the bottom side of device (not bezel) in the left and right corners it should have two "laptop security holes".
On the bottom side of device the HDMI plug is centered.
On the right side of device the headset jack (top corner), AC plug (center) and USB plug (bottom corner) are present.
On the top side of device on the right corner there is a stylus slot that slides towards the right.
On the top side of device the SD slot and microphone hole are centered.
On the left side of device the Volume rocker (top corner), lock button (centre), USB plug (bottom corer) are present.
On the back panel, the battery, removable, is centered and covers a large surface

The tablet can be locked to a desk by the "laptop security holes"
The tablet can be connected to a keyboard+mouse+battery accessory.
This accessory has a swivel hinge on the base and the keyboard+mouse on the top. It attaches firmly via the "laptop security holes" and connects via a USB.
This accessory swivels to form a clamshell (ie closes) or opens up to a maximum angle to double as a kickstand (refer to the "Litl").
This accessory can be attached backwards (keyboard side down) to retain the tablet form but provide extra battery power.
Another accessory is identicle in form but it accomplishes as a dual(touch)screen to provide more screen-estate and enhance the experience by providing a new means of interaction.

4)
Darwin (OSX) is out of the question, I am not Apple, nor do I want to risk legal/Hackintosh problems
Windows 7 is the best OS available but it requires a powerful CPU, has security risks and only runs on x86 CPUs
Windows CE/Mobile/Phone are possible but not appealing OS's due to age and restrictions and the lack of versatility (contarary to x86 Wins).
Linux is great! It is safe, highly functional/adaptable, light and runs on most architectures. The problem is that it is not popular as personal computers as the others and lacks finger friendliness.
However, Android is popular and cross-compatable Linux but lacks the flexibility or compatability to its (Desktop Linux) father.
MeeGo if essentially Android (has all of its advantages) but (none of its weaknesses) is very versatile and can be compatible with Desktop Linux applications.
The clear winner is Windows for powerful device and MeeGo for weaker one.
Since MeeGo has not been officially released it is quite a significant gamble.
So infact the obvious answer is Android vs Windows.

After considering both systems I would prefer Windows 7 which is highly tuned to run quicker and has undergone an innovative custom skin surgery from the ground up.
If I cannot accomplish to get fluid/decent speed and apply a finger-biased skin, it will get the job done and have access to the largest libabry of Apps and constant support.
It is very versatile and should make the diy guys/nerds appeal to it aswell as the casual user (something they are used to/something compatable with their pre-exisisting pcs)
If MeeGo project doesn't gain speed and the adjustments to Windows 7 are inadequet, Android (pref. Froyo) would win by default.

5)
In this case, this slate should hold roughly (4-cell) 5,200mAh underestimated slightly.
I think 7hours is sufficient for mobile use.
I'm not familiar with many CPUs (mainly Intel and ARMs) but here's a sample:
5.2hrs with Intel Core i7 640-UM Dual 1.2GHz*
5.7hrs with Intel Core2Duo SU7300 Dual 1.3GHz
7.8hrs with Intel Atom N450 Solo 1.7GHz^
8.8hrs with Intel Atom Z600 Solo 1.9GHz^
9.0hrs with ARM Cortex A9 Dual 2.0GHz*
*similar performance, ^similar performance, All figures guestimated according to target device by exsisting devices or research.