I'm surprised no one has mentioned this yet.. Elon Musk also founded PayPal and SpaceX. While I don't think Apple plans to buy a spaceship or even an electric car company, I certainly wouldn't be surprised if their meeting was centered around paypal and mobile payments.
Another possible point of discussion (although probably less likely given the size and power-output of batteries for electric cars versus portable electronics) would be Tesla's battery technology. Tesla has done some pretty innovative stuff with Lithium-ion batteries.
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