Originally Posted by
The Swampster:
“I know - my kids have them. 
The poster posed the question - what's the use of buying a tablet? I suggested some answers. I am not a gamer, but if gameplay was as enjoyable and satisfying on a DS (which has the advantage of being portable), as it is on a full-size TV screen, nobody would bother getting a Wii/Playstation/XBox.
A tablet-size product is still pretty easy to carry around (similar to a book), but the screen size is arguably "better". (Well my kids seems to think it looks that way, anyway.)”
They all cater to different types of gaming.
The DS is for casual gaming, or for kids in their bedroom without access to a "bigger" system.
PSP - casual gaming, maybe a little more serious than the DS
iPhone/Touch are definitely for more casual gaming.
The Wii/PS3/360 are more traditional, getting people around the bigger screen to play.
When I used to commute to London I'd swap between the PSP, Symbian phone, Nokia 770 and iPod for music/gaming/video based on capacity and what I was interested in at each particular time.
How a tablet fits into this is anyones guess though. Its a lot larger than a casual gaming system, doesn't have anywhere near the power of a dedicated system.
Microsofts attempts at doing tablet variants of Windows have done practically nothing, but then Microsoft don't often have much success in trying to create new markets. Or even getting into markets controlled by one other major player.