atomica offers this usage note:

Assure, ensure, and insure all mean “to make secure or certain.” Only assure is used with reference to a person in the sense of “to set the mind at rest”: assured the leader of his loyalty. Although ensure and insure are generally interchangeable, only insure is now widely used in American English in the commercial sense of “to guarantee persons or property against risk.”

i tend to almost invariably use and expect 'ensure' unless a specific warranty is expressed or implied. i'm betting sparteye can provide some insight here.