As with its new iPhone, Apple has consistently delivered an approachable product that many users find intuitive to operate.

We're talking about the seemingly abstract but critical subject of user interfaces, about which it's clear that innovation has only begun.

What will we see as we move past keyboards and mice to run our machines?

Think about hand-held products, and contrast the iPhone with the Amazon Kindle.

Friends who know my interest in electronic books ask why I don't own a Kindle: The answer is that it lacks a touch screen. Find a word whose definition you need, and you're forced to triangulate on the word using hardware controls rather than just pointing at the word to highlight it for the lookup. This kind of touch screen, with a single input from stylus or finger, is common to hand-held devices such as the Palm TX, which is why I use it for e-book chores.

But touch screens are evolving in interesting directions.

What stands out about the iPhone is that it is the first widely adopted and wildly popular multitouch technology whose history goes back decades in various research labs. I use a stylus to highlight words on my Palm TX, but multitouch accepts input from several sources, which is why you can leaf through photos on an iPhone, select one for examination and use finger gestures to enlarge or contract it.

Fingers do the working

People like using their fingers to make things happen on a device.

I talked to a Kindle owner at a concert last winter who, like me, lamented the lack of touch on the Kindle but wasn't interested in a stylus. "I lose every stylus I have," he said, "but I've never lost a finger."

True enough, and also true that with multitouch, actions once performed with a menu and a series of taps or clicks become remarkably intuitive. It will be interesting to see how PC makers begin to work tactile feedback into devices.

That kind of feedback -- it's a science called haptics -- might be useful in controlling robotic tools or simulating complex situations, as in flight-training devices or medical simulations.

But before we go too far down that road, we need to see how touch technologies play out and where they are most useful. Despite the euphoria of finding new ways to input data, shoehorning a task into a technology isn't always the optimum solution.

There are times, for example, when an on-screen keyboard simply won't do. If I'm writing at length, I need a true keyboard with space to write unimpeded.

For that matter, there is a price to pay for any interface that forsakes the tactile and replaces it with a smooth surface. I have problems with glare on many PC screens, but they are nothing compared with using a hand-held device in bright sunlight. Data input is a problem when you can't see the screen, a factor when you're trying to do something as basic as changing the song selection in an MP3 player without taking the device out of your pocket. Thus a tactile alternative via external controls can be a useful backup.

To see multitouch technology at full throttle, look at Perceptive Pixel (www.perceptivepixel.com). Founded by researcher Jeff Han, the company uses large screens with multitouch that readily functions with multiple users. Now we're talking interactive brainstorming that can be extended over a network, using touch to manipulate and visualize complex databases of information.

Be ready for multitouch to ignite numerous startup efforts as software emerges to capitalize on the technology.

We know that Microsoft, which has already delivered a touch product called Surface, is planning to incorporate multitouch in the version of Windows that will replace the ill-regarded Vista. Opening the operating system with the largest market share to multitouch acknowledges the fact that usability is the key to a good interface.

Looking at these changes, I can't help but feel that although we are talking about different types of user input -- mice, keyboards, touch -- we're also sketching out a future in which computers will more or less disappear.

The digital interface will become so natural that it combines the senses of sight, sound and touch into a seamless computing web, so that we go from computing as a thing we do with a particular machine to computing as a mesh of ubiquitous information accessible through many devices and even everyday objects. Multitouch is one herald of that change, but it's hardly the last.

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