The definition of churn rate was the subject of a shareholders' lawsuit against Netflix in 2004. The shareholders accused Netflix of reporting an artificially low churn rate using:

  Number of Customers churn / (Number of customers at beginning + number of customers gained). 

Where the plaintiffs preferred:

  Number of Customers churn / (Number of customers at beginning + number of customers at end of period)/2. 

Netflix succeeded in having the suit dismissed, since there is no official way to calculate churn.

http://www.globenewswire.com/newsroom/news.html?d=62086

http://www.docstoc.com/docs/33875708/In-Re-Netflix-Inc-Secur...