As I stated in the above-linked question, general Bluetooth communication to external devices on non-jailbroken iOS devices is restricted to MFi-compliant Bluetooth hardware.
However, newer iOS devices (iPhone 4S, new iPad) are capable of Bluetooth 4.0 LE communication with external devices without the need for those devices to be MFi-compliant. This interaction is done through the new Core Bluetooth framework, which lets you send and receive arbitrary data to and from Bluetooth LE devices. This only works with those listed newer iOS devices, though.
Tim points out an interesting hack that you might be able to get away with in making your device appear like a Bluetooth HID keyboard. Devices like this barcode scanner have special modes to appear as HID devices to iOS. You might be able to pull something together based on this, but all data transfer will be one-way from your device, and it looks like this will require entering that data into text fields as if you had a keyboard connected.