The DS would make a very good platform for an adventure game. Since adventure games are a fairly relaxed experience in general, which is ideal for when you're just burning some time on the train and may need to get off at any moment, it'd be a really nice to way to kill some time.
The touch screen interface itself would be absolutely excellent too, opening up the possibility of just tapping the screen where you want your character to walk, tapping on objects and such, and of course the touch screen pad would open up some interesting new minigame-type puzzle oppertunities.
The only drawback is the size of the screen. To really work, the actual game itself would have to all be on the bottom screen, which can admittedly get a little difficult to see clearly on -- particularly if there's a lot of detail. The top screen would end up being wasted, possibly useful only as an inventory view or something.
I really wish they'd made the DS with a bigger screen. It'd probably bump up the price slightly, but it'd make games a lot more fun. It just seems a little too small right now, and it obscures a lot of the detail in its games, not allowing it to demonstrate its true power.