Maximum PC isn't typically known for running Mac rumors, but since the vast majority of iPhone users also run PC's, it might interest you to know that Engadget has uncovered shots of an iPhone 4G prototype , and they are pretty convinced they are legitimate . The low-resolution photos are strikingly similar to leaked iPad shots which also turned out to be genuine, and the fact that the same iPad can be spotted in the background only serves to seal the deal. The evidence sounds pretty flimsy to me, but given that these guys deal with gadget rumors 24/7, I'm inclined to give them the benefit of the doubt.
In addition to the photos, the Engadget source also revealed a few revised hardware specs that fall in-line with expectations. They claim the device sports a high-resolution display, forward and rear facing cameras, and support for MicroSIM cards (the same type used by the iPad). Of course even if this turns out to be true, its still just a prototype that could be drastically different from the final product so take it for what it is, a promising lead. On the other hand, iPhone announcements typically happen sometime in June, suggesting that this could indeed be a fair representation of what to expect.
The pictures don't tell us much, but they do suggest now would be a bad time to run out and buy a new iPhone.