I very much doubt what you are hearing is correct. IS and GS owners are very different in terms of target market. IS appeals to a younger market vs. a more mature GS owner. I have driven both cars and although I enjoyed both cars they are quite different given the GS is more refined. Being more refined loses in my opinion some of the fun factor.
In short, everything I read indicates they will have an more extended IS line - maybe they will be based on the same/similar platform as the new 2006 GS. Like Audi A6/A4.
