GameSpot. The game is being co-developed by Korea's Neowiz Games, maker of EA Sports FIFA Online. There's a chance that MMO (see!) could be related to Capcom's recent Mega Man Universe trademark filing, but if it is, the company isn't saying. When contacted, a rep told Joystiq, "We have no comment at this time as Capcom does not respond to rumors and/or speculation."
Whatever MMO turns out to be, we can only hope it doesn't take after Ghouls 'N Ghosts Online, Capcom's last (and unreleased) attempt at turning one of its classic IPs into a massively multiplayer experience. It was scary ... bad.