Eactly as Ysss said, its a ever-changing term - what power user today will definitely be different from tomorrow technology. And its really a subjective term - I hardly think you can strictly define it. But just understanding the full potential of your device and using it to the hilt would be what a power user does (unlike say using a phone to shoot photos only or phoning with a camera.)
They're maemo and MeeGo... "Meamo!" sounds like what Zorro would say to catherine zeta jones... after she slaps him for looking at her dirtily...