A paper in today's issue in the journal Nature, puts to rest the idea that large parts of Titan's surface are currently covered in methane/ethane oceans. Several years ago, a paper by Campbell et al. in Science, presented evidence for wide-spread liquids on the surface of Titan from specular reflections seen in radar pulses sent from Arecibo. The paper, West et al., in today's issue of Nature uses near-infrared observations near 2 microns. Like Campbell et al. they used ground-based observations to look for specular reflections along a wide-range of longitudes near 25 South Latitude. They found no evidence of specular reflections in their data. Specular reflections at near-infrared wavelengths would be much more conclusive for liquids on the surface than at RADAR wavelengths. For specular reflections to be seen, a surface must be smooth at the scale of the wavelength used to observe the surface. So for RADAR specular reflections, a surface only needs to be smooth on the order of 13 cm (or 3 cm for Cassini RADAR), which can be achieved by such terrain as a mud flat that contains only small, smooth rocks (think Huygens landing site). For a near-infrared wavelength specular reflection, terrain has to be smooth on the order of 2 microns, which is VERY difficult to achieve with anything but a liquid surface. So the absence of a specular reflection at near-infrared wavelengths would seem to rule out liquids at 25 South latitude. The presence of a RADAR specular reflection may indicate that some of these areas are covered in mud flats (or regions without large boulders or rough terrain), perhaps indicating that liquids were present in the past.
There are some pathological cases where you could have a liquid covered in floating crude, but very few materials can float on liquid methane, except perhaps acetylene, but it still has to overcome the low surface tension of methane. Additionally, this study only looked at liquids at 25 South. This leaves open the possibility that liquids may exist seasonally at the poles as they receive enhanced rainfall.
UPDATE: 08/04/2005 12:45pm: New Scientist has a more in-depth article
online for those without subscriptions to Nature.