That was due to a current limitation on dag system.. that leads to cyclic dependencies.
So now you can attach the origin of a simpledeform to an empty and mark whether its
affected by the relative position of empty or the global one.
brecht found a workarround to solve the cyclic dependency by
using a parent of both the mesh and the control empty:
empty_parent
|
|-> empty controlling simpledeform
|-> mesh with simpledeform modifier
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