http://hackaday.com/2013/04/26/cube-...bulk-filament/
detailed explanation here: http://www.howmuchsnow.com/cube/

Purely physical solution. NO messing with electronics or potentially buggering up your warranty.

No doubt a firmware update will be released in the future to prevent the printer printing with an empty cartridge. There's an easy solution to that - don't update the firmware :-)