Seems that this is still the case with 2012?
I found that
1. if I draw a spline manually, it is editable.
2. if a fit an existing sketch to a spline with "fit spline", it is then not editable, button like "Add tangential Control" "Add Curvature Control" all failed.
You can't add tangency or curvature control to a Fit Spline because it is already defined by the underlying geometry.
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