Based on the rumors of features and the discontinuing of the iPod touch 4th gen. I can determine this iOS 7 full support at WWDC will be:
iPhone 5, iPod touch 5th gen, iPad 4th gen. and iPad mini
Limited support will be:
iPhone 4, iPad 2 and iPad 3.
I say this because the devices with FULL support would also support AirDrop for iOS, due to the broader dual band Wi-Fi, which aren't in the limited support devices. Other features like the design, Flickr and Vimeo, and iRadio will be on the limited support devices. Dropped devices will be the 3GS and iPod touch 4th gen, based on the discontinuation of the 4th gen, for an A5 device, This could mean the A5 will have a longer lifespan and start to be limited around iOS 8.