Just got an 'Integral' brand MMC Mobile for my 770 - it says "Compatible with RS-MMC" on the blister pack.

Plugged it in and moved the Ice Age 2 trailer to the card (took about 15 secs to move the 8MB file).

Now that I have the movie playing off the card (as I type this ), I think we have the MMC Mobile question well and truly settled. They work!