One example of such a site is Meetic, which started as a free dating site for a couple of months and then started charging a subscription.
How do you prevent customer backlash? How do you protect yourself against a competitor jumping in with a free alternative?
I think as long as you make it clear from the start that your site isn't going to be free forever, you won't get any significant customer backlash -- simply because those people who would provide said backlash wouldn't sign up in the first place. Whether this is an effective business strategy is an open question, but it certainly seems like the most honest approach.