I'm in the process of trying to get Blender 3D fully supported for content creation for IMvu and this little problem is the final hurdle in that... (almost.... there... Scotty... we need.... more power!), most everythng works (after *a lot* of work) except for this issue - bones can lay flat and objects in IMVU will do that same, upright and they do the same *except* on the '-Y' axis, they get squewed. The good news is that they all get squewed by the same angle so they all appear in IMvu with the same rotation along this axis. Some shots below to clarify the above.
(below) The bones as they are orentated in Blender - 'Y' = up, 'Z' = pointing into the room

(below) The orientation of the bones show in top down view in Blender.

(below) The results in IMvu

So the above shot shows the bones on the wall to the left ("2") behaving correctly, whereas the bones on the wall to the right ("1") do not, despite being correctly setup in Blender. If the bones are rotated to compensate it doesn't straighten the calander up (so it lies correctly against the wall as "2" does) but instead squews them at an odd angle.
I've been able to dublicate this problem in another Blender file and exported mesh/rig; same problem, same results (which *is* kind of encouraging as it means the root cause is the same thing in all instances).