An "unguessable" URL like pages.example.com/dc87b967-90d1-42cc-808f-e789ecae45c3 is approximately the same as a password-protected page with a simple URL w/a single global password distributed alongside the URL. That is to say, not really secure but unlikely to be stumbled upon.
Other than that, to truly make things private requires some sort of audit and expiry infrastructure which isn't available unless you run things through your own servers. Oh, and running over SSL and not sending passwords over email.