iPodObserver has published a photo what appears to be a new revision of the Apple iPhone 3G.
The photo shows the bottom portion of the iPhone as pictured here. You can also see a new model number: "A1303" and that model number has not appeared on any released iPhone to date.
According to the post, "The new iPhone case back appears to shed its glossy appearance for a matte black look, and also sports what may be etched markings which indicates the plastic used in the current iPhone 3G model may be dumped in favor of a metal back -- much like the first generation iPhone."
iPodObserver was the first to post this picture, but additional pictures were later leaked to Macrumors and you can see those here.
This isn't the first piece of speculation (see this article) regarding an iPhone hardware refresh in June.