If it was dual-voltage (DV), then in all likelihood it should say at least "DV" on the card, or "MMCmobile" (or minimally on the package, but I'd think it should be on the card, too).
Dual-voltage RS-MMC cards support both ~1.8V and ~3.3V operations and the 6680 can only handle the 1.8V. That's why regular 3.3V-only RS-MMC cards won't work.