There has to be some really "intelligent" text input system for it to work well with a flat surface. Else you'd have to be looking down at your fingers while typing.
Some kind of tactile feedback would be nice. I wonder if we will see something like TactaPad built into multitouch diplays in a few years.
The lack of tactile feed back is why I don't think we'll see a multitouch keyboard on any thing much bigger than an iPhone or maybe a small tablet. In such a small area, you can see the keys as well as the text field you're typing in.