Tinhte, who claim they not only have a white iPhone 4 from Apple, but it's one with a "test version" of iOS that nobody else has yet seen. It's difficult to ascertain how legitimate this software is -- it could just be a neatly done jailbreak mod -- but that site has a track record of getting its hands on Apple gear ahead of the pack. With that said, the multitasking menu shown here substitutes the current use of apps' icons to represent them with a visual of each app's open window. You can tap on a window to expand it (replete with animation) to fill the screen or long-press on it to bring up the familiar "x" button for shutting it down. This is all accompanied by a new "Search iPhone" dialog at the very top, which sends you into Spotlight search that looks very much the way it currently does (though it seems to no longer be accessible with a left swipe from the first homescreen as on previous versions of iOS). Jump past the break for the video.
Update: A second video has been uncovered, this one showing that we're looking at a prototype unit and taking us on a tour around its body. A visit to the Settings menu shows a 64GB storage capacity, while app folder creation is also handled a little differently from iOS 4. The presence of the Touch Fighter app on this phone, which Apple built to show off the capabilities of earlier versions of the handset, and other internal-looking software seems to point to this indeed being some form of iOS beta build. Of course, it might not be iOS 5 at all, but simply a never-released version of iOS 4. All we know for sure is that the video's after the break.
Update 2: All indications are that this is indeed an early, unreleased version of iOS 4. Interesting, but probably not something to get your hopes up about.