An update from Polyphony was expected to give GT5 Prologue the chance to wreck some cars, but now they reveal they're "around the bend and down the straight".
The team's primary goal right now is "making sure GT PSP (and GT5)" gets in the hands of gamers, so content revisions to Prologue are likely canned. PSP will have indestructible cars.
In response to a fan question on the US PlayStation.Blog, Polyphony said: "Sorry FoomMan, we're around the bend and down the straight: the team's main focus at this point is making sure GT PSP (and GT5) are in your hands."
GT PSP is due for release this October with no planned DLC, though it's connectivity feature to PS3's GT5 "does leave it open for you to imagine the possibilities". A downloadable demo for GT PSP is a wish of Polyphony devs "but at this time, there are no plans."