It depends on your state labor laws. In the past, California exempt employees had to be paid for a full work week even if they just worked one hour due to protect their exemption status. However, the California DLSE just recently issued an opinion letter reversing their earlier stand; however, it is a sticky arena for California employers.
On August 19, 2009, the California Division of Labor Standards Enforcement (DLSE) issued an opinion letter confirming that under California law, as with the federal Fair Labor Standards Act (FLSA), salary reductions which accompany reductions in hours does not jeopardize employees’ exempt status.
I blogged on this topic:
It doesn't appear that you are in California but I would check with you state labor board...