Dual SIM on the iPhone 4
Because I have a personal phone and a professional one, I decided to try a double-SIM adapter for the iPhone. I got one from [here]. The iPhone does not have to be jailbreaked ; mine runs the stock iOS 4.3.5. It had to be unsimlocked though as my personal provider is not the same as the one from work. Namely, it worked with both Bouygues Telecom and SFR. You cannot have both SIM available at the same time.