Distance is a major, but not sole determinant of stability. An orbit that is too heavily perturbed by the Sun, at the edge of a planet's Hill Sphere will eventually be wrenched free of the planet.<br /><br />However, in Titan's case its distance from Saturn has no bearing on its temperature. Saturn gives off some heat, but certainly not enough to warm any of its moons or rings. The volcanic activity mentioned is less a result of distance and more a result of gravitational interactions and orbital resonances among the moons which could distort each others' orbits, resulting in tidal stretching. This is the case with Io and Europa at Jupiter.<br /><br />Theoretically, any body inside the Roche Limit could be torn apart; this is what would happen to Saturn if Titan moved too close. However, this only applies to objects that are held together primarily by their own gravity--large, spherical moons like Rhea, Dione, Titan, etc.--rather than small bodies held together primarily by electrostatic forces, like Pan, Prometheus, Janus, and other close-in moonlets.<br /><br />By the way, William K. Hartmann's excellent <i>Moon and Planets</i> (fourth ed.: Wadsworth, 1999) gives a basic formula for calculating the Roche Limit for objects "larger than about 40 km in diameter, orbiting icy or stony bodies of modest strength" (p. 60):<br /><br />1.38 * (rho <i>M</i> / rho <i>m</i>)^.33 * R, where:<br /><br />rho <i>M</i> is the density of the primary<br />rho <i>m</i> is the density of the secondary (moon)<br />R is the radius of the primary<br /><br />The answer will be in radii of the primary.