Polyphony Digital is going a step further with pre-order bonuses for Gran Turismo 6, as detailed on the PlayStation Blog. It's not all retailer-exclusive, thank goodness:
But, yes, there's plenty of that as well with GameStop offering 16 extra vehicles (full list below) and other goodies including the Beyond the Apex II app. This 15th Anniversary Edition Upgrade is faster than individually unlocking the featured cars, which also happen to be present on-disc. On that note, Amazon's bonus for pre-ordering is literally $1 million in GT6 credits.
The 16 cars in the Anniversary upgrade are: