I know a guy who built a floor sensor using the QT products. He had some panels engineered, about 25Cm square each, with 100 touch-sensitive spots on each. Worked great. Had them manufactured. Looked great, top quality and excellent engineering.. or so it appeared.
It turned out that when he put a grid of these panels together ( it was a floor sensor) the fields of QT chips interacted with each other. Many of the panels ended up in the trash, they were unusable for the original purpose. So you should be aware that the QT products are pretty carefully engineered; if you try to make them do things they weren't specifically engineered for, like work together in a grid, you will have problems. (....The engineer responsible for the above design couldn't fix it.)
Have you considered using conductive foam sensors? You could make a sort of grid. Also there is a company in Victoria, BC, Canada called Tactex that makes conductive fabric. The conductive foam solution works great for touch sensing. See the Playground exhibition section, the one called "customers design with their fists".
Finally, a good rule of thumb in designing this kind of thing is to ask if you have ever seen such an interface available commercially. this will tell you if the technology is stable enough and cheap enough to manufacture and have it actually work. I haven't seen anything bigger than maybe 100 sq cm that is really touch sensitive, have you? Experimental technologies are sometimes trouble because the "experimenting" take $$$ and lots of time. Even for a hobbyist thtis can mean $500 and fifty days... So you have to be smart about your design from the beginning, and realize your capabilities, and ask how much time and money you have to spare.
This is one of the interesting side-effects of the Arduino project: it makes electronic design so modular that something that even two years ago would have been beyond the reach of a hobbyist now appears to be simple
. A lot of things are
simple with Arduino, but for really complicated things like a touch-sensitive grid array, you are still going to need the $$$ and time, time and more time. You just have to remember, in the case of things like the QTProx sensors, orsay, an Ipod, that many hours of fine-tuning and preparation have gone into making it look simple. If you want to change that simplicity to another design, it's probably goign to be very complicated.
PS: I feel ancient, but I hope i don't look ancient