After purchasing a few sets of dishes that were covered in grey utensil marks, I discovered that scrubbing them with 0000 steel wool (the finest steel wool sold at most hardware or paint stores) and ceramic/glass top stove cleaner cleared away about 70-80% of these marks.  This has worked especially well on some heavier sets of dishes such as Pfaltzgraff and Dansk.  It will also work on finer porcelain too.  I've never encountered any problems using this method however I would emphasize that you use 0000 steel wool, the finest steel wool available.  This method will not remove knife scratches that have actually cut into the glaze but it will remove the majority of grey marks left behind by stainless flatware.  I would also caution against doing this to any pieces that have a decoration applied on top of the glaze.  Most decorations are under the glaze and will not be harmed by this however if a piece has been painted or had a decal applied after firing, this method could harm the decoration.