Is the Bluetooth 4.0(Ble) backwards compatible?
Bluetooth Low Energy (BLE) is one aspect of Bluetooth V4, designed specifically for setting up simple, low power, links between devices. A BLE link requires a V4 device at both ends. V4 'High Speed' is basically a, bluetooth negotiated, adhoc Wi-Fi link, and requires V4 devices at both ends, I believe. V4 'Classic Bluetooth' supports the legacy bluetooth profiles, and is backwardly compatible with previous version devices.
The BLE shields so far available are just BLE, and don't provide 'High Speed' or 'Classic' support.
The real issue with iPhones prior to the 4S is that Apple don't provide SPP support in the iOS SDK.