Well looks like Kazunori Yamauchi (head of Polyphony) might have made a bit of a blunder. He was giving an interview where he was asked
As good as Prologue is, what can we expect in the full version of GT 5?
"To be honest, I haven't given a lot of thought to GT 5, so I don't have an answer. And it's too early to talk about GT 5 because we still have updates coming out for Prologue.
Is there an ETA on GT 5?
It won't be this year and nothing's scheduled for next. It may be out in 2010, but like I said, we haven't really discussed it yet.
http://www.importtuner.com/features/impp_0811_gran_turismo_5/page1.html
So looks like the rumour about prologue being rushed out to just get a game out there to compete with the other consoles racing games could be true. The worrying thing is that Gran Turismo is the main franchise for Polyphony and if they havent given it any thought and nothing is scheduled for 2009 that basically could mean the game is in early development and could be even longer away. Could end up being late 2010 or even 2011.