I really don't think it's going to come to "Family Guy without Seth" like so many people are thinking. Most likely the 'near completed' episodes are ones where even the ADR has been done and Seth's involvement comes down to signing off on the final edits, sound mix, etc. Nothing that can't be done by a higher-up if say, Seth had a sick day. It's just a formality for him to approve it, since it's his baby.
If it gets to the point where entire segments aren't completed and McFarlane is required, I'd think Fox would put the show on hiatus rather than continue. And if there's a fallout between Fox and McFarlane after the strike's over, the show would probably be A)canceled or B)wrap up production and then not be renewed. Unless FG has some unbreakable contract through 2010 that I haven't heard of.