BBCnews is carrying this report on a new investigation triggered by the Huygens probe on Titan. According to all standard models, the methane on that moon should be completely destroyed by UV light, perhaps in as little as 10 million years. Something is replenishing it. While geologic processes can account for it, it's also quite possible life of some sort (almost certainly microbial in an underground ocean) is also responsible.

The good news is the Huygens data should be able to provide at least part of an answer. The bad news is, the replenishment is probably caused by "serpentinisation", a kind of geothermal activity.