Just going to throw in my two sense here.
Pocket PC and Palm devices have had touch-screens for YEARS. And yet, if you look at the games available for them, the touch-screen is RARELY used, or is simply used as a pseudo-mouse. I'm not entirely sure what it was that Nintendo thought would be so innovative about putting one in the DS, but the platforms that have been available until now haven't seen much in the way of unique gameplay. I certainly don't discount the possibility that there is room for improvement and imagination, but there is a fair amount of history working against it.
Jonny has a point about your hand being visible when you're playing, which makes it hard to go on autopilot. But for me, I find that my hand is, more often than not, obscuring the screen. In fact, I'm not entirely sure that this isn't why the games on Pocket PC that use the touch-screen aren't turn-based or low-action games!
Quite frankly, I'm kind of surprised that Nintendo would miscalculate in this way, which leads me to beleive that they've got something planned on the first-party side. Unfortunately, the strength of a platform is almost always measured by the third party support, and the slow-trickle of games at the present time is hardly reassuring.