I discovered this difference in an old http://www.dailyfrenchpod.com video podcast episode (one of my favored French podcasts). Being familiar with the sound of the commonly used word soir (evening), in which the s is pronounced like the English s, I was surprised to discover that in rasoir (razor), the s is pronounced more like an English z, which I confirmed with my Larousse English/French dictionary app.
Not that my having pronounced it incorrectly in Paris over 30 years ago prevented the saleswoman from understanding my request:
Vendez-vous les rasoirs jetables?
(Do you sell disposable razors?)
4/1/14: I had forgotten the word besoin (need), whose s is also pronounced more like an English z than an English s! The s in soin (care) is, just like the s in soir, pronounced like the English s.