Would be nice to have something official posted here at PCars Official site. Hopefully this is true, hopefully someone from PCars can Comment on this. But still no pre-order on PSN Store in USA.
Here is what someone said they got as a reply from Slightly Mad...

"""
Hey [iceman741],

Thanks for your email.

It doesnít look like weíre going to have a preorder on the US Playstation Store, sadly. It turns out that for preorders to be accepted on the Store, we need to apply for a spot eight weeks in advance. Because of the moving release date, we were unable to do that. Once the game had gone Gold, we felt it more important to get the game out there rather than delay just to lock in preorders.

Unfortunately, we canít secure a slot now. Lord knows, Iíve tried. I know itís a pain in the neck, and we are doing what we can Ė but our powers are limited! Really appreciate the support, though.

Once the game is available via the US Playstation Store, bear in mind that our digital versions come with both the Limited Edition and Modified Car Packs Ė so youíre not losing out on any preorder content.

Thanks, and hope to see you on the track, Pete.

TL;DR - No preorder, but we still get the car packs. Sigh of relief.

""""

Hopefully this is true, would have been nice for the studio to have just said this from the "beginning" so to speak.