The 4 day work week?

A story in the Star recently examined how some Canadian companies have taken a liking to the concept of cramming 40 hours into 4, 10 hour work days, rather than the more common 5 day work week.  This isn’t a new idea.  It’s been floating around for decades as a possibility that might improve employee morale (by giving permanent long weekends) and efficiency (for example, by lowering overhead costs or reducing the number of shift turnovers).  According to the story, the 4 day work week is attracting renewed interest because of rising fuel costs, which make it more expensive to keep machinery running and raises the cost of employee commutes.
The story focuses on Daimler-Chrysler, and notes that the company is in discussions with the Canadian Auto Workers, which represents the workers.   Depending on the language in a collective agreement regarding hours of work, a unionized employer may need to bargain a change from a five-day to a four-day work week.  What about a non-union employer?  Do you think a non-union employer could introduce such a change?
It’s an easy question if the employment contract gives the employer the unilateral right to change hours and work schedules.  Likewise if the employee agrees to the change, and many might.  Presumably, consideration would flow both ways under such an arrangement, so an agreement to change hours and days of  work would be enforceable.
How about if the contract is silent, and the employee doesn’t want to change her work schedule?  Perhaps the employee has family responsibilities that prevent her from working 10 hour days.  Do you think that a unilateral change in a work schedule from 5 to 4 days would amount to a constructive dismissal?  Should the employer be permitted to make this change without the employee’s agreement?