I've had the same problem.  Spike had a small spot on his tail a few months
ago, and it took over the back half of his tail about a month later.  I took
him to a vet (who didn't seem to know a lot about ferrets) and he said it
was an adrenal thing.  He gave me a bottle of shampoo and told me to use it
on his tail ONLY once a week.  (I was supposed to use the normal shampoo on
the rest of him since the stuff is somewhat harsh...)
 
It seems to be working so far.  The name of the stuff is Seleen (Selenium
Sulfide USP Lotion) and is used on dogs for removing skin debris associated
with dry eczema, seborrhea and non-specific dermatoses.
 
Hope this helps...
 
If anybody has any more information on what causes this, please let me
know as well. I've been a non-visible reader of the FML for about 9
months now and regard the opinions here to be more credible than somebody
who treats pregnant mares for a living...
 
One last question: Spike is stone deaf. Is this normal for white ferrets?
 
Thanks in advance for the advice!
 
Robb Inniss