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. You have to choose one over the other. To activate a SIM, you have to open “Settings”, “Phone” and “SIM Applications”. From there, just select the SIM you want to activate. Uncheck the “auto/SW” option if you don’t want the hardware to switch from SIM to SIM on low signal. Since you switch SIM, you get a “No SIM Card installed” message. Wait for a few seconds and you should get the signal. I know when it’s OK since I have PIN codes on both SIM cards and the hardware asks for it as soon as I switch.
As I can’t use both SIM in the same time, I setup an automatic forward from the professional phone number to my personal one. Then, when I need to phone from my pro line, I just switch to the pro SIM. The case itself is made of hard mate plastic. It’s really light and doesn’t turn the iPhone into an ugly big brick. Final thought: quite a nice piece of hardware for “only” 16€ ; would have really been perfect if both SIM could be enabled at the same time.