I think we can imagine a federation which provides its own currency, one which isn't bounded geographically like nation-states. Precursors (from the corporate world) could be those e-money places, or maybe even things like frequent-flier miles.
Of course, this does not necessarily preclude localized currencies... And I'm not particularly thinking of a given economic system like mixed capitalism or whatever.
A reasonable currency which isn't limited by geography may have efficiency advantages.
Tayssir