The Peel PG92 not only provides dual-SIM capabilities but packs an additional battery as well. Unfortunately it appears that the Peel PG92 will only support jailbroken iPhone 4 or iPhone 4S as it will require the download of a customized app that mimics the default dialer interface of the iPhone.
The case also will not support the micro SIM card that is used by both the iPhone 4 and iPhone 4S, but instead relies on a mini SIM. Unfortunately no word on pricing or availability , but there is a sign up page on Vooma’s website that will notify you when the Peel PG92 dual-SIM case is released.