From the American Ferret Association –

The American Ferret Association highly recommends that all ferrets be properly vaccinated against canine distemper and rabies under the care of your veterinarian. With this in mind, due to the high incidence of reactions each year, we also believe it is necessary to do further research to determine if the possibility exists that we could be over-vaccinating our ferrets.

http://www.ferret.org/news/07-april-titer_study.html

From the The Rabies Challenge Fund -

Many dog caregivers across the country are becoming more and more aware of the possibility that over vaccination can have harmful effects on dogs – and the rabies vaccine is no exception. The incidence of rabies exposure in companion animals and humans, though very low today, still remains a distinct possibility. Consequently, the Federal and state governments require every dog to be vaccinated triennially (still even annually or biannually in a few states) against rabies. However, the veterinary community is increasingly concerned that rabies vaccination given this frequently is not only unnecessary but also responsible for inducing adverse reactions that lead to both acute and chronic immunological and other disorders. Although a French medical study completed in 1992 showed that rabies vaccine is protective for at least five years, Federal and state legislatures have not accepted this finding.Thus, the health and well-being of millions of dogs in the United States remain threatened by the potential for adverse vaccinal events.

http://www.rabieschallengefund.org/index.html

Ferrets are susceptible to rabies, but it is a rare disease in this species compared to dogs and especially cats. Part of the reason for this is an inborn resistance to rabies. The occurrence of rabies in closely related wild animals, such as weasels and mink, is very low. Ferrets have very thick, tough skin, and not all bites will penetrate. For rabies virus to be transmitted, the skin must be broken and the wound contaminated with saliva from the rabid animal. There have been fewer than 20 cases of confirmed rabies in ferrets in the USA since 1958; several of these were associated with vaccination of the animal with a live vaccine not approved for ferrets. No human being has ever been reported to have contracted rabies from a ferret.

http://www.peteducation.com/article.cfm?cls=11&cat=1283&articleid=562