I'm aware that cycles doesn't provide any deforming blur but it does in the internal render, the problem is that if the samples are different than the shutter value then the render results into a weird solution with multiple times the object repeated in different positions, check the attached file. It should render fine as the samples and the shutter are both set with the same value but as soon as these valuer doesn't match then the render gets weird.
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Confirmed, it doesn't seem to be a new bug either, strange how this was not noticed earlier.
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Fixed some issues in svn now which make this work, but the .blend file needs a change too. What's being done here is that there is a shape key for every frame. This works fine on exact frames, but between those it's using bezier interpolation which caused the shape key weights to not add up to 1. If you go the graph editor and change the interpolation type to linear it works, together with the fix I committed.