Depending on who you ask, they might even tell you that an interface that needs tool tips to be understandable needs to be redesigned, badly (cf. Jef Raskin: The Humane Interface).
In fact, tool tips are a solution to a very specific problem: Iconic buttons with no labels, such as seen on toolbars. Whenever you have labels, use them. No need to provide a tooltip because you already have text to tell what a particular control is going to do.
What's more is that touch interfaces map not very well to today's WIMP interface model. Many controls are good to handle with a mouse pointers but are frustrating to use with a finger. Menus, checkboxes, radio buttons spring to mind. So the interface paradigm for touch interfaces has to look rather differently to today's mouse- and keyboard-driven interfaces.
So I think it's not so much a lack of tool tips that's the problem here but rather that we didn't explore many new ways of interacting with a computer in the past 30 years (basically not since the research done by Doug Engelbart and Xerox PARC in the 60s and 70s).
Touch input is just similar enough that it kinda works for most purposes. But it not only lacks a location-without-touch component but also precision. Basically all touch input is good for is touching something and dragging something. Even double-tap is difficult so what we really need is some fundamental change in how to design and craft UI specifically for a touch interface.
You'll see some of this in dedicated devices, such as the iPhone simply because that's a platform that neither has a mouse pointer nor a keyboard and only touch. This means you don't have to build a UI which has to be usable with all possible methods of interaction (a problem with plagues Windows currently; I do have a multi-touch laptop but for many many tasks touch just doesn't work) but only with one. But a general-purpose solution for "normal" software and computers is pretty far off at the moment, I think.
So I'd advise you to just think a little different about how you design your UI. As said before (and can be read in Alan Cooper's About Face), tool tips are for labeling controls that don't have labels or where space wouldn't suffice to place them. Key usage scenario here are tool bars. But an interface designed for touch would make all controls larger anyway. Many small icons, closely grouped together are a pain to use with touch input even if you had tool tips, simply because it lacks precision.
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