Was there ever a resolution to this issue? I have code essentially identical to that in this thread, and I am now experiencing the same problem with ListFeatureClasses leaving out feature classes for particular feature datasets. I am working with a file geodatabase, v10.0 (no service packs--see note below). In my case I have one feature dataset that has one of ~30 feature classes whose name contains the name of the feature dataset. I get no feature classes from that dataset. The fGDB was just thrown at me to work on, and I don't know the provenence. I suspect, however, that it was exported from SDE.
Notably, there are other anomalies happening, too: (1) I get no feature classes from another dataset that has just two feature classes, neither of which contains the name of the feature dataset; and (2) In a couple of other datasets, most of the feature class names end with "_MT" or other "_XX" or "XX" (like "ZM") letter combination. In those cases the feature classes are listed, but the "_XX" (or "XX") suffix is deleted, leaving just the portion of the name to the left of the suffix. The only other thing of note is the lack of service packs on my system. I had SP2 installed, but that service pack introduces a bug (now reported as such by ESRI Support) that breaks a lot of my existing scripts. I had to uninstall SP2. Anyone have any further info on this problem. Chris? Thanks for the help.