You assume PD makes logical decisions, when in reality they have a long history of not having their priorities straight. They failed to manage even one project properly, and now you're suggesting they are working on two completely different projects. Though I wouldn't put that past PD (given their history in weird decision making), it would most likely mean we'll never see either, or two half-assed results.
We know that the PS4 will have a completely different architecture than the PS3. We also have indications (hinted by amar212) that GT6 will be a PS4 game. Combined with PD's history, I suspect we won't be seeing a new full GT game for a long, long time.
Many people here seem to think that PD learned from their mistakes and did a complete 180-degree turn in their development process. These kind of changes (and I've seen them quite a lot in software product development) are generally very disruptive and require huge effort to be executed properly (and usually fail the first time, because people rarely respond well to being forced to radically change the way they work). While certainly possible, we haven't seen anything that confirms such a radical change.
More likely, I think they are still providing updates for GT5 because they love their product and keep on tweaking/tuning it until the next game comes out. But I think it's just tweaking/tuning and they're not preparing for a GT6 release on PS3.