If you ever wanted to get some dual SIM action going on your iPhone 4, or iPhone 4S, now you can. But you're going to need to take the jailbreak plunge first.
The system is on offer thanks to the Vooma Peel PG920, a battery back up case that also has room for a regular sized SIM. Combined with the jailbreak-only Vooma app, you can then switch to use your secondary SIM card - essentially using your iPhone as a slave UI to the PG920's phone capabilities.
It's not a perfect system as yet, TechCrunch tried it out and said the interface is a little clunky, and it's only in pre-release stage at the moment.
The US-based site did describe it as "a very cool and clever hack" and one that "works without problems on any jailbroken iPhone" though.
In terms of looks, think along the same lines as the likes of the Mophie Juice Pack. Sure, you'd be adding a bit of bulk to your iPhone's skinny frame but you'll get a battery boost too, along with the added SIM functionality - handy if you're travelling abroad, or you're a drug dealer or an adulterer.
If you are interested in the Vooma Peel PG920, you can register your details on the company's website. Vooma states that the PG920 is FCC approved.