What is in the CE that isn't in the standard?
Standard Edition:
Just the game.
Collector's Edition
The Collector's Edition is exclusive to the USA, Europe and Asia. The Collector's Edition for the USA includes the game, a custom etched Keychain, a 1:43 scale model of a Nissan GT-R with exclusive livery, a 300-page APEX book with hints on driving technique, car tuning, future technologies and more, an e-voucher for five unique "Chrome Line" edition cars, and a Certificate of Authenticity. The Collector's Edition has sold-out at retailers GameStop and Amazon in the USA; it is however available at other stores in the country. The European Collector's Edition will include the game, a 300-page APEX book with hints on driving technique, car tuning, future technologies and more, an e-voucher for five unique "Chrome Line" edition cars, an exclusive GT5 dynamic theme and a Certificate of Authenticity. The Asian Collector's Edition will include the game, a 1:43 scale model of a Mercedes-Benz SLS AMG with exclusive livery, a 300-page APEX book with hints on driving technique, car tuning, future technologies and more, and an e-voucher for five unique "Chrome Line" edition cars.
Signature Edition
The Signature Edition is exclusive to Europe, Australia and New Zealand. It will include a steel case finished in 'Obsidian Black' (official Mercedes-Benz SLS AMG colour choice), a coffee table book that details the cars and locations in GT5, a 300-page APEX book with hints on driving technique, car tuning, future technologies and more, a Gran Turismo-branded wallet containing a competition entry card giving one person the opportunity to win a real Mercedes-Benz SLS AMG car (the competition will be similar to the GT Academy), a GT-branded USB pen drive containing an exclusive GT5 dynamic theme and a Polyphony Digital/Mercedes-Benz exclusive trailer, a custom etched Keychain, a 1:43 scale Mercedes-Benz SLS AMG model in an exclusive livery, an e-voucher for six unique "Stealth" edition cars and an e-voucher for five unique "Chrome Line" edition cars.
Sourced from Wikipedia, double checked with gran turismo's official website.