IMHO, this will not happen. Although I agree that it could save the company money, as Asqx already mentioned, there is not the infrastructure to support the addtional workforce of 757 and 737 production at Everett. Hell, the area can barely handle the widebody workforce it already has. Anyone who has ever tried going from Lynnwood to Marysville between 2:30 and 6 in the afternoon knows what I'm talking about. A standard 15-25 minute trip at non-peak times becomes a 2-3 hour nightmare. Imagine if you increased the workforce by 50%. Doing constant staggered shift changes might allow it to remain constant, but it would make that nightmare last all day. Plus, you must consider parking at the plant, flight line availability, impact on the local airport (Paine Field is a large recreation flying airport), and of course, housing for the employees who have to move.
IMO, Boeing should look at building a new plant for the 737/757, as well as they're eventual replacement, at Grant County airport in Moses Lake. This area has plenty of room to expand, although I must admit, it wouldn't be an ideal place to live.
Honor the warriors, not the war.