The system currently operates in Montreal, QC; Ottawa, ON; Melbourne, VIC; Minneapolis, MN; London; Washington, DC; Toronto, ON; Boston, MA; and Chattanooga, TN, plus some other special locations. It opens tomorrow in New York City, which is why this idea came to mind. I don't think I'm alone in visiting multiple cities in that list in any given year.
Call it "Global Membership" or even "Gold Access" or something. You'd issue a special key that unlocks a bike in any station in any city in the system. Play up the "VIP" angle and make the key a bright gold or something. I think they'd sell some, even if it cost more than just getting temporary access in other cities when you visited, simply because of convenience.
Ultimately it would be very cool if one's bikeshare membership worked everywhere there was a bikeshare system of any type, but I'll certainly settle for being able to get a bike anywhere this particular system operates.
ETA: The B-cycle network, a competing system which operates in cities across the United States, rolled out support for memberships across 15 of the cities in its network back in March.