I think they delayed it in order to collaborate it with the release of 3D TVs (3D TVs are coming Summer 2009, and GT5 was shown in 3D).
While possible, that seems a tad silly to me. 3D HDTVs will probably be very expensive at first and not many people will have them. Spending time implementing a feature most people won't see or use doesn't seem too wise. Sure, more people years down the road would have access to it, but I still don't buy it.