Working in Blender 2.70, I've created a simple model (a box with a few loop cuts) with a simple armature (2 bones). I attached the mesh to the armature and created two animations (actions) and called them 'ArmatureAction' and 'walk'. With the 'ArmatureAction' active and the mesh and armature selected, I export the model.
Here are my export settings:
When I open up the .x file, I see two AnimationSets: both called 'ArmatureAction'. However, if I export with the 'walk' action active, I get two AnimationSets: 'ArmatureAction' followed by 'walk'. Edit: In this case, the ArmatureAction animationset is made up of 865 lines whereas the walk animationset is only 289 lines. The walk animation has fewer keyframes, but I thought that it exported every frame, regardless of whether it's a keyframe. In that case, the discrepancy might indicate an issue.
Does the currently selected action make a difference when exporting the file? I'm using Assimp to load these files into another piece of software and I'm having trouble getting the 'walk' animation to show up at all.