Multitouch capacitive screens and pads are arguably the first new style of input and device interaction to hit the mainstream since the gamepad became the standard for most modern gaming consoles.
Even if we most associate multitouch with the Apple product ecosystem at this point, by itself that's still an enormously high-profile "niche."
HowStuffWorks describes the technology of the iPhone's touch screen and one of Apple's relevant patents, for a mutual capacitive screen, can be found here. A mutual capacitive screen includes two active layers; one contains lines with a driving current, the other contains sensing lines. The location of a touch or touches by a conducting object (such as a finger) can then be detected because of its interaction with the electric field generated by the drive current.
Interpreting multiple touches and their associated movements as gestures is a complicated software exercise, but it all comes down to decoding points of contact and the paths they take as a function of time.
An obvious attraction of capacitive touch technology is that it can be operated directly with the fingers, so no special accessories such as a stylus or puck are required to operate. And if you do want to use a stylus, say because it's more natural for writing, one can be used as long as it's conductive. (A stylus, such as the Pogo from Ten One Design, has a metal barrel and a tip made of a conductive foam commonly used to protect electronic components.)
However, capacitive screens are not without their downsides. For one thing, you can't operate them while wearing full gloves. At the same time, you have to make sure that you only touch the screen with the body parts you want to write with. Some artists working with the iPad wear gloves without fingertips for this reason.
A variety of approaches have been applied to detecting and processing touch over time. These have often involved using a special stylus or puck (an object similar to a mouse in shape) with a special tablet. Best known today through Wacom's products, these devices generally work as follows although the details will vary by device:
Under the tablet's surface (or LCD in the case of the Cintiq) is a grid of wires that transmits a send and receive signal. In send mode, the tablet's electromagnetic signal stimulates oscillation in the pen's coil-and-capacitor circuit. In receive mode, the energy of the resonant circuit's oscillations in the pen is detected by the tablet's grid. This information is then analyzed by the computer to determine the pen's position. In addition, the pen communicates other vital information, such as pressure-sensitivity, side-switch status, tip or eraser orientation and Tool ID.
Resistive screens and tablets are also commonly used in tablets and mobile devices and are ubiquitous today in kiosks, ATMs, and other displays of that type. The advantage of resistive touch is that, while a stylus may be used to increase precision, any object (including a finger) works.
This brings us to the question of which characteristics will be important in touch-sensitive phones and tablets as we move forward.
The tablet that has everyone going gaga these days really is quite a nice piece of gadgetry; I bought one and have no regrets. It's great for Web surfing, reading the news, and even its soft keyboard is more effective than I expected. Certain types of drawing even seem to work pretty well, although that's a judgment I leave to those who can--well--draw.
But it's not very good for taking handwritten notes. A stylus and a program like Dan Bricklin's Note Taker HD makes note-taking on an iPad practical by letting you write in a magnified window into the page. But it would be hard for me to describe handwriting on iPad as either "magical" or "revolutionary." This, I think, is an interesting fact, given that historically mobile tablets (including Apple's own Newton), have tended to focus on handwriting and other forms of writing directly on the screen. In fact, the difficulties associated with doing reliable handwriting recognition often get cited as a reason that tablet PCs have never really gone mainstream.
Essentially, the iPad has deprecated handwriting as a primary mode of interacting with a tablet.
I'm not convinced that's the future. Google's Tim Bray rightly points out that input is one of the big limitations of mobile devices. Writing isn't a panacea, especially in a world where many people are increasingly comfortable dealing with miniature keyboards both hard and soft. But, in an environment where input has to largely take place through a single sheet of glass, it's a bit hard to believe that high resolution writing and drawing don't have some role to play.
And that's going to favor technologies that may let us just use our fingers for pressing and gesturing but also open up the possibility of more precise work using some handheld tool.