SMS GV Extension, a popular jailbreak tweak that merges Google Voice SMS messaging capability with the Messages app, has at last received unofficial iOS 6 support. The tweak enables iPhone users to send SMS through Google Voice alongside the built-in MobileSMS module, the latter delivered by cellular carrier.
The jailbreak package is available on Cydia for iOS 6 by adding the repository http://cydia.angelxwind.net/ under Manage > Sources > Edit > Add, at least until it appears on the mainstream ModMyi or BigBoss repositories. The tweak has no icon, but rather is configured through Settings. A reboot is required after installation.