I think it means someone that heavily relies on multiple features of a particular device, and maybe one or two not so obvious features of that device. The general concensus would probably be the users that are early adopters of new features and those that develop new use cases for these devices.

The power users of the PC in the 90's is the power user of the smart device today.

Honestly, though, there will be a new segment created by this device. The power user doesn't apply to the devs. I'd consider them the trailblazers and mavericks of the market. (I prefer Mavericks myself. GO DIRK!!) The "tinkerers" will be a new market, and the N900 is the only game in town. All other devices need another device for making apps, but not this one.