Overtime restrictions aren't a production constraint. Foxconn will just hire even more people and add them to other shifts.
Production constraints for Apple are likely going to be supplier based, or just overall factory capacity.
Overtime restrictions are a production constraint. Especially with the current labor shortage in the Pearl River delta, where Foxconn has 2 factories. Add in the high turnover rate at Foxconn, and its faster to produce many iPhones with seasoned workers working overtime, compared to newbies working short shifts. There was an interview with a Foxconn worker in March that demonstrated this--they're rate of production was better with seasoned workers that had worked overtime.
Factory capacity is tied to overtime restrictions. They need to build more dorms if they're going to double the workers. They also need to build more factories if they're going to churn out 50-60 million iPhones for the launch quarter (they sold nearly 40 million just for iPhone 4S launch).
In the end, its cheaper for all parties to start earlier, instead of building more factories, more dorms, and hiring more workers. They'll do a combination of both, of course, just because of the insanely high volume expected. Start 2 months earlier or hire double the workers and build new dorms and factories? Start 1 month earlier?