Man, why on earth would they NOT release it on PS3 soon, and PS4 later?
On PS3, it will be a big update (GT5Spec3): basically new cars and trackes. No new features at all. Easy to do as we all know they already work on cars/tracks for years.
That would bring them at least 50 millions $ guys... just for some cars and tracks. The only people involved are the modelers and sound engineers... who would basically work for PS3 and PS4 at the same time. (they will obviously use PS3 assets on PS4, as Kaz already said: their models are future proof... and they probably have versions of cars that are even more detailed than the photomode ones... from the very start)
PD programers are already working on PS4 for years! They were even involved in its conception.
So why on earth would a company not take the 50 easy millions $ (do you realise that it would help finance the cost effort due to the PS4 production/price?)...and, one year later, release GT6 on PS4? In all its glory.
Only smart choice here.
Especially as, since Forza, GT is not such a big system seller anymore. People used to have only one choice when they were racing sims fans: GT. Now they have another one (even if it is still behind GT in numbers, features, graphics, etc...): that's enough to neutralize the Gt-momentum. I m pretty sure it was MS's goal in the first place: not to beat GT, just to give an alternative: to be able to say: "we have it too". Which is the case: simple fact.
So... GT5Spec3 on PS3 very soon...
And GT6 on PS4 at the end of 2014.
Mark my words.