The New Scientist has an article online regarding new insights from the GCMS instrument onboard the Huygens probe which landed on Titan last month. After examining the isotropic ratio of Carbon-12 and Carbon-13, the GCMS team surmises that the carbon in Titan's atmosphere (in the form of methane) is replenished since the ratio does not show signs of fractionation. Fractionation is the process by which lighter isotopes are preferentially lost from an atmosphere compared to heavier isotopes. In the case of Titan, Nitrogen and Oxygen show fractionation (in the case of nitrogen, severe fractionation) but carbon does not. So in this case, methane is replenished at a rate greater than or equal to the loss rate, while oxygen and nitrogen are not. An interesting result that does have implications for the current state of Titan.
Thanks to imran for the heads up.