iOS 6 Maps flyover will require A5 processor
Apparently, the 3D flyover feature will require the power of the A5 processor (at least) in order to run properly. This means that the Maps flyover will be available on the iPad 2, new iPad, iPhone 4S, and future mobile iProducts, but it will not be available to those with original generation iPads, iPhone 3Gs or iPhone 4, although those devices will still be able to run iOS 6.
If you're still stuck with an iPhone 3Gs, you'll also apparently find that you don't get access to a few other iOS 6 features like FaceTime on 3G and VIP Mail lists, though we're not really sure why VIP Mail lists would be a feature that gets left out. It doesn't seem like something that would be all that resource intensive. Still, the best option with mobile devices is to not hold on to that old hardware too long, because eventually you're going to get left out.
source: Ars Technica