Unfortunately we cannot simply use existing transform for rigidBodyNode, because we should keep Maya shape node(and its transform) as well.
It may be needed, for example, for high-quality rendering.
Now rigid bodies have just very simplified polygonal rendering.
Also we cannot just copy all data from existing transform node, because Bullet requires that origin of a rigid body coordinate center should be in its center of mass.
So I copied data from the resulting worldMatrix attribute.
Get the dynamicaUI.mel file r2051, I think this should solve your problem.