I'm way too optimistic about Todd's "shortest wait between announcement and release" statement. Since we've had what seems to be an incredibly long wait, I'll say we're playing the game by next summer. Wouldn't it be cool to have some kind of teaser in FONV? Probably won't happen since it's Obsidians baby, but I can dream.
I don't think it will be released any earlier than Fall 2011, personally. But that all depends on when it's announced.
Do you think the 'other big game' they've got in the works is FO4?
I'm very confident that it is. I don't think they're going to make any new IPs for a while, and are just going to work on TES and Fallout for a few years. After Fallout 4 is released, if they have a big enough team, I can picture them starting work on a new IP, but for now I think they feel TES and Fallout are enough.