Apple leaks say that there are three different prototypes being tested for the next iPhone. The weirdest one seems to be one that looks like an iPhone 4, but has a case that slides out to reveal a hardware keyboard. The slide-out mechanism sounds similar to the Keyboard Buddy, is pretty unlikely, since Apple has pushed the touchscreen-only train so hard the past few years.
The other two prototypes are said to be exteriorly similar to the current-gen, maybe just 1/3 smaller. If the rumors are right, we shouldn't expect a huge aesthetic change, just improvements like an 8-megapixel camera and an upgraded battery.
It's also interesting to note that there are three different prototypes at this stage of the game (February). It could be that Apple still hasn't made up their mind about which unit to release. If this were April and closer to when they needed to test the final design more thoroughly, we'd guess that two of those three prototypes are just decoys, made to throw people off from what the real iPhone is, to avoid what happened with the iPhone 4 last year. Perhaps that's what's happening already?