Some dreams come true...

[TheKaneB]

Mathematica needs a proper keyboard to be used at its maximum, otherwise there are things such as WolframAlpha for the more casual use with a smartphone.
I'd say that my earlier convertible laptop / tablet with Windows on it was the closest thing to a portable device capable of running Mathematica. You might find smaller devices on chinese sites which run Windows and are as small as you can get. They will technically run Mathematica but I doubt about their actual practicality as a work machine.