The Minneapolis City Council recently passed an ordinance that prohibits the use of electronic cigarettes in indoor public places, including places of employment, effective immediately.  In addition to workplaces, the ordinance will apply to other public areas such as stores and restaurants.  The only exception is for  electronic cigarette sampling in exclusive tobacco shops (including exclusive electronic cigarette shops) that restrict people under the age of 18 from entering.
In support of the ordinance, a press release from the City explained that:

Studies have shown that electronic cigarettes and their vapor contain toxicants and nanoparticles that may increase the risk of cardiovascular and respiratory disease. There have been no long-term studies conducted on electronic cigarettes, so the lasting impact on the health of users and those exposed to their vapor is unknown. While many people report that electronic cigarettes have helped them quit smoking cigarettes, electronic cigarettes have not been proven to be cessation devices, and the Food and Drug Administration has not approved them as one.

Other cities in Minnesota that have similar ordinances include Bloomington, Duluth, and Edina.
Even in cities that do not prohibit electronic cigarettes in the workplace, employers may adopt their own policies that prohibit them.  Potential reasons for adopting such a policy include the unknown health effects of electronic cigarettes, co-worker concerns, and the difficulty of enforcing a ban on traditional cigarettes, but not electronic cigarettes.
Takeaway:  If they have not already done so, employers in Minneapolis should implement policies that prohibit the use of electronic cigarettes in the workplace.