From the PlayStation.Blog:

We’re finally able to announce that our latest free downloadable Burnout pack bikes will be out 18th September!!!

It’s tough to give specific dates on downloadable content, because we don’t know for certain until the last minute. But hey, we’re really excited about bringing bikes to Burnout, so we’re going to take a chance and tell you that we’re aiming for September 18 – that’s this coming Thursday!

There is more details on that post, but nothing new. The question is, get it now, or wait until the game is released on the PSN as a downloadable game?