I believe Siemens demonstrated something similar for mobile phones. I think it's not going to be possible any time soon, simply because of the power requirements. If you want to project something onto a surface, even if it's small, you have to use a certain minimal amount of power to be able to see that. And even if your projection system doesn't lose any energy, that's still a tall order for a battery that would fit into a pen. So it's a nice idea, but I think for the next couple of years we'll be stuck with lugging large projectors around and having to plug them into main power - and with old-fashioned mechanical keyboards.
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