For iPhone 4 can do plenty, even play nicely on other GSM networks when you've unlocked it with ultrasn0w, but up until now you couldn't put two SIM cards inside it.  For iPhone 4 Dual SIM Adapter Case doesn't quite change that – the two SIMs in question aren't exactly for iPhone – but it does allow you to easily flip between cards without reaching for the Liquidmetal tool.

That's because each SIM clips into a special adapter that then snakes round the side of the iPhone 4 and into the regular SIM tray.  A transparent plastic shell keeps everything safely in place, and there's no need to cut the cards beforehand