Although the iPhone 4, iPad 2, iPad 1 and even iPhone 3GS are capable of doing turn-by-turn navigation and Flyover in iOS 6, those two features are exclusive for iPhone 4S, new iPad 3, and the upcoming iPhone 5. Any iOS device older than the iPhone 4S and iPad 3 will not get turn-by-turn navigation and Flyover. Apple clearly states on their iOS 6 Maps page that these two feature will be excluded from older devices:
Some features may not be available in all countries or all areas. Flyover and turn-by-turn navigation will be available only on iPhone 4S and iPad 2 or later. Cellular data charges may apply.
Sadly, Apple has used this marketing ploy once again to make user upgrade devices if they want to completely have all features iOS 6 will offer; like was done last year with Siri. But hopefully — like what happened with Siri — the jailbreak community might up come up with a tweak or app to enable urn-by-turn navigation and Flyover on unsupported devices.