The iPhone’s most elusive app to date is about to sweep back onto the scene — but don’t look for it in the App Store, and you won’t even have to jailbreak your device to get it.

Google Voice remains one of the most controversial apps for the iPhone. While Google themselves had Apple slam the door in their face before they even got to the App Store, a couple of small developers did manage to bring Google Voice to the iPhone, albeit briefly.

In addition to Sean Kovacs’ excellent GV Mobile (which is still available for jailbroken devices), developer Riverturn was also there early on with VoiceCentral. Both apps were pulled from the App Store on the heels of Google’s own denial, and the resulting firestorm even set off an FCC investigation that is still ongoing.

None of this sat too well with Riverturn (check out that full story here), so they’ve set out to change the rules by coming up with VoiceCentral Black Swan. They call it “the next revolution in iPhone applications,” meaning it’s not tied to the App Store nor is it a website optimized for the iPhone (which Google has been rumored to do for Voice since they were shut out). Riverturn calls Black Swan a feature-rich “weblication” that enables you to view and manage your Google Voice data just like a native iPhone application, combining the device’s easy-to-use interface with the convenience of an always-up-to-date web application.

Among the features promised for Black Swan are a native iPhone look & feel, the ability to sync Google Voice data (including importing Google Contacts), offline access to Voice data, the ability to listen to Voice messages and read transcriptions and even swiping to delete calls, voicemails or SMS conversations.

Black Swan is currently in a very limited beta, but if you already have a Google Voice account you can sign up to be part of the beta program or just to be notified when Riverturn begins accepting new users. Of course, an existing GV account is required, as well as an iPhone or iPod touch running iPhone OS 3.0 or higher. We’ve already registered for beta access, so stay tuned to this channel for more on Black Swan.