Maybe it could be like the multi-touch. The problem is there's no tactile feedback to it, you always need a reference point of some kind, to indicate when you want to actually interact with it. With multi-touch the actual touching acts as the reference point for that.
You know, as corny as this may sound I think the mouse's sucessor will probably come from Apple, before Microsoft. Apple is a company that has the user experience at the helm, they look at the interface in ways that can enhance the user experience, Microsoft is not at all like that.
Even from a distance I can already tell a bunch of practical things that don't quite work with something like Natal as a sequel to the mouse. I'll wait for other companies to show their solutions, I wouldn't doubt Apple has something in the pipeline. They already patented an IR pointer remote controller interface, which frankly probably works much better once you get it in the consumer's hands, than something like Natal.







