The problem I am having may be related to the one described here:
http://forums.planetside.co.uk/index.php?topic=5386.0but there are differences.
I am working on a scene, no extreme settings but a few populations, again nothing extreme; a few thousand instances each. From a complete re-start; Windows (XP or Vista), and TG. Even a low resolution, low quality test render completes but with a completely black image and no errors or warnings. Just in case this is a memory issue I have restricted TG to using one core; no help. If I disable a population of Eucalptus trees made by JCinBAMA the scene renders correctly. If I leave this population but disable all the others It renders black. I don't think it is the object as I have tested that by rendering a separate scene containing a population with many more instances than I need without error.
I can complete my scene as this population is not vital but it is interesting that a single population, one of many can cause a problem.