Company Fires All Employees Who Smoke

Michigan Firm Won't Allow Smoking,
Even On Employee's Own Time
UPDATED: 8:55 PM EST January 25, 2005
LANSING, Mich. -- Four employees of a health care company have been fired for refusing to take a test to determine whether they smoke cigarettes.
Weyco Inc., a health benefits administrator based in Okemos, Mich., adopted a policy Jan. 1 that allows employees to be fired if they smoke, even if the smoking happens after business hours or at home.
http://www.wral.com/news/4126577/detail.html

This brings up all sort of interesting legal and ethical questions for the employee as well as the employer. (Fair) enforcability would be the obvious difficulty with any such policy. What about privacy? Does the employment contract give up such a "right"?