Construction started a month or two ago. There are pictures on various sites. Disney was projecting a 2014 opening. Latest information is early (1Q?) 2014.
I don't think anyone really knows the construction schedule. I agree with the PP, it certainly could be done by the end of the year if Disney follows Universal construction schedule with Transformers and schedules overtime. Some reports suggest Universal is working 24/7 on Transformers.
Universal wants to open Transformers for summer of 2013. Give people a reason to visit Universal for the summer season. Does Disney need to give guests a new reason to visit during Christmas?
I wouldn't plan a trip anytime in 2013 if the purpose of the trip is to see 7 dwarfs mine train.
This is the rumors and news board. At this point a 2013 opening for mine train is a rumor (speculation) and not news. Doesn't mean it won't happen but don't base your plans on it happening.
edited to add--I'll speculate at least part of the attraction, the gift shop, will be open in time for the Christmas crowds. As a PP said capitalize on Christmas crowds.