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It's a gimmick. Can you imagine doing any amount of work at a desk with your arm extended out to use a touchscreen? That's a recipe for RSI.
Yes I can imagine it. I used it and it works great. You don't sit there with your arm out indefinitely. You reach down and type. Sometimes you use a mouse or a trackpad, and sometimes, you reach up and do things on the screen. If you haven't tried it for real, don't knock it. As I said in another thread, it's actually good to move your arms around a bit. Maybe then they wouldn't get so flabby on a lot of people and we wouldn't have so much carpal tunnel. Of course, it's not for everyone. Some people liked those ridiculous IBM pointer buttons in the middle of their keyboards. I couldn't stand them.
Like LTD said, current desktop OSes are not designed for touchscreens. As much as you think that's what you want, it's just not viable at this point. Be patient though, it's a concept just waiting for someone to get it right.
Windows 7 is partly designed for it, though it's a bit of an afterthought. I would hope Win 8 will be much better at it. I had a recent Acer touchscreen laptop that was NOT a tablet and I liked it, but it was not a Mac. I'm sure if Apple would put their minds to it, they could do it right. I wish they would implement at least basic touchscreen support, like Windows, so those of us with touchscreen monitors on our desktop could take advantage of some of those great Lion gestures right on the screen and not just on the trackpad.
And yes, I have an iPad too.
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And that's because design is always a set of compromises and priorities. The OP asked for a "touchscreen computer." Meeting that requirement calls for considerably more horsepower than the iPad (or any tablet) can currently provide.
It's not a question of manufacturers failing to recognize that a market for a touchscreen computer exists. It's that providing real computing power and features along with touchscreen capabilities is not just a question of sending the requirements to the factory where it can be magically produced.
Touchscreen ability doesn't require any extra computing power. It's just another form of input like a trackpad or mouse. So instead of thinking turning IOS into a real computer, think adding touchscreen to OSX. The key is designing the OS to be more fat finger friendly. And it doesn't have to be built into the "original" computer hardware - there are some fantastic, relatively low-cost touchscreen desktop monitors out there (my Acer T231H is a prime example) that could be used right now with Mac OSX....if they only built that capability into it. It seems to me it wouldn't be that big of a leap since they are already moving in that direction with the trackpad gestures.
The original post did say portable, though, so for me, the the MacBook Air would be a perfect candidate (which would of course require a hardware change to make it a portable touchsreen computer). I find myself reaching up and touching the screen all the time after using my iPad. You don't have to have it in a tablet configuration, although that would be optimal. As I've carried my MBA around, I've imagined it as a touchscreen, with the screen just folding all the way back on itself, and then I sigh.