Vooma gained attention last year for its Peel case, which bought phone capabilities to the iPod touch, and now it's launching a similar case to make your iPhone 4 and 4S dual-sim capable. Along with a 1380mAh battery, the clip-on case also holds a GSM SIM slot and unlocked cellular radio. To use to use it, you'll need a jailbroken phone loaded with an app available on the Cydia store, which interfaces with the radio to provide a second phone line. While the case is bulky, it's no worse than other battery packs, and the curves might even improve the ergonomics of the phone. Although dual-sim handsets haven't taken off in the US, they're popular in countries where having two providers can mean access to reception far more regularly.
TechCrunch has already got its hands on a pre-release version of the device, and says that once it's set up the PG920 "works seamlessly". The applications controlling it offer a similar look and feel to Apple's stock dialer and messaging apps, but aren't fully integrated into iOS — you'll need to keep both your stock apps and these custom ones. There's no word on price, availability, or which GSM bands the Peel works with yet, but you can sign up for more information on Vooma's website.