I have a lot of bluetooth devices connected to my iPhone and iPad. Mostly headsets, speakers or keyboards.
They are getting really good these days. For example i love my logitech slim keyboard for the iPad or my new Parrot Zik headset.
No Wire, No Fire...
The problem is simple though.
Once you have paired a bluetooth device to more than 1 destination device (let's say your iPhone and then your iPad or your iPhone, your wife's Phone and your computer), you can't easily decide and control which of those 2 or more devices are going to get connected first. For example my i will turn on my headset and i will not know if it will connect for sure to my iPhone first or iPad first.
The Only real way to turn bluetooth devices you don t want to get connected and then turn on your Bluetooth device with the destination device left.

Bad experience
Some have brought a solution by creating patterns to switch from one device to another (by example by double click on a button). But
1. you have to remember which button/pattern
2. it does not guarantee you that the next device you will connect is the one you want (e.g. if you have 3 devices connected like 2 iPhones and a Mac which will be the next one?)
The only good way i found so far was created by Logitech on this Wireless Keyboard which has 1 click access to 3 different devices: 1 button for each 3 device. You press #1 you are connected to device 1, #2 you switch to device 2 and #3 switch to device 3
And i guess this is how this should be solved, until it is cheap enough to build for each bluetooth device a digital display which will enable that elegantly (maybe by touch or swipe)

Someone listening?