We've been told this couldn't be done (making SW backward compatible), but we are getting models from a vendor that are SW version independent. What they are doing is sending a macro that builds the whole model from scratch. I'll have to say somebody did some pretty fancy programming to get this done but it sure works slick.
As an added bonus, the macro can be emailed and is quite compact.
In comparison I've down loaded from 3D Condiment Central with the 2004 checkbox checked and have gotten parasolids which were useless for my purposes.