Some 6 months ago, I was able to download an app by the name of OnOff before Apple removed it from the App Store. I simply tap the app icon and my iPhone connects to my car's Bluetooth. When exiting my car, I tap the said icon and the phone's bluetooth shuts off. This icon sits on my home screen.
Why Apple removed this app is beyond me. In the app's review section, it was mentioned, by some, that this app would eventually be removed and it was.
Why stop at bluetooth. Apple needs widgets for many toggles. Wifi would be more important for me.
I don't know why they haven't implemented a toggle bar in the notification center or in the spotlight page. I'm sure it will come eventually, and will be touted as revolutionary and take up a slide or two in the keynote.
I believe since bluetooth 3.0 (the 4S and 5 have 4.0) is so efficient and is dormant when not in use, that is not worth toggling on/off. Perhaps not even wifi. At least not along the same lines that GPS, LTE, or 3G drain battery life.
The new iPod Nano has bluetooth. Haven't read up much on it since it just came out, but if it gets days of battery life that should be a good indicator bluetooth on the iPhone doesn't cause battery loss.