It wouldn't surprise me if Sony is pushing PD to have GT5 released as soon as possible (next Spring), with the bad economy and all. And if that is true (I love a big guessing game) than PD has put all their focus on the full game and no longer on Prologue, hence the puny spec III update and no spec IV on the horizon. PD have stated earlier that Prologue eventually would be (through updates) as mature as GT5 (gameplay wise) and that the difference between the two would only be in the number of tracks and cars and game layout (timetrails, events, races etc.). If PD has indeed the intent to release the full game next Spring, than we might get all the tracks and cars (these are supposed to be completed, except for some off-track details), but we might at first not get features like private rooms and damage. Those we might get through updates as we would get other updates like more tracks and more cars and ....
So, my guess is that GT5 will be released next Spring with a lot of cars and tracks and for the rest will be not much more than (or as much as - I'm quite pleased with it as it is now) Prologue as we have it today.