sbml to cellml converter
Matt has done the basis for this in a chunk of work he did converting celldesigner models to CellML models. This does not include the math.
A key is to maintain enough semantic information for the roundtrip back into SBML and also to not use reaction elements.
Where SBML use content mathml, the transformation is not so bad. Where SBML refer to a particular kind of mathematical formulation and simply represent the paremeters, then we need to put together the stock math pieces that we will use to support that.