There's something about the photo below that doesn't quite ring true, doesn't the Xbox 360 on the right look unnaturally large? Could this just be the angle the shot has been taken from? Nevertheless, we thought we'd post it anyway and let you be the judge of that.
Rumours of an Xbox 360 slimline version have been doing the rounds ever since the news that Jasper, the codename of the latest Xbox 360 consoles, in which the existing ATI graphics chip has been shrunk to 65 nanometre, hit the shelves. That smaller chip means that Microsoft could, if it wanted to, fit the Xbox 360 innards, which would also include a smaller memory chip, into a smaller chassis.
Considering Jasper is being rolled out as we speak, and we haven't heard anything about an Xbox 360 slim, we'd guess that it's never going to happen.

Source :: Engadget
UPDATE - We've just spotted this, posted on the blog of Paul Thurrock, the guy who took the shot. It was the angle of the photo that made the one Xbox 360 look slimmer and NOT an Xbox 360 slim. (Thanks Daza from the forums).