I think the "mystery fantasy project" was Dungeon Siege 3 all along (as "demonstrated" by the fact that Feargus was the project director on it for a time. Wonder if the change was made when the game entered another phase in production...), this new one is fairly recent I think.
As for who's on each project, I think it's like that :
- Sawyer and Taylor are on New Vegas and Dungeon Siege 3, that much is known.
- Avellone, with all the work from New Vegas, is probably on a project that isn't too far along, I would say Wheel of Time.
- Lyon is probably on that "private" project.