I agree with Johnathan. On a cost/GB basis, CF cards are definitely not cheap and I wonder about the practicality of it. You wouldn't have to purchase too many 'backup' CF cards before you get into the cost of a P2000 or some such similar device. Bigger storage, more versatile, the list goes on. From there, DVD backup or portable HD. I can't really see any advantage to backing up from one card to another, in the field or anywhere...
Mike.