Can a Bluetooth 4.0 BLE device talk to a Bluetooth 2.1 device at the most basic level?
There isn't much information about the bluetooth/radio chip embedded in the newer iPhones (4s, 5, iPad 2, newer macs), but it is commonly known to be a Dual Mode Bluetooth 4.0. They can access older Bluetooth (v1.x) Bluetooth + EDR (v2.x) and Bluetooth + HS (v3.x) devices, as well as newer Bluetooth v4.0 AND BLE v4.0 devices.

The host chip on the iPhone can talk to both, but not at the same time. A BLE slave device can only talk to a BLE host. A BR/EDR device can only talk to a BR/EDR Host. There can be dual-mode slaves, but not really.

Yes, you can use your iPhone with Bluetooth v4 to talk to a basic BR/EDR device. You can't use a iPhone with BT v2.x to talk to a BLE device.