Nobody thinks that having four icons indicates that they finally made the smaller iPod Shuffle-based iPhone? There was speculation last year that they were looking at a smaller lower-priced entry iPhone with limited features based on the Shuffle. I would wear it running, so I'm hoping the four-icon picture isn't just a coincidence.
That would, indeed, be awesome. I take the iPad almost everywhere, so I have no need for an iPhone which is little more than an iPad Nano. Thing is, I need that "little more" - the phone part - as well, and want it as a separate device which I _can_ have everywhere. It's back to the "just a phone" desire, a device focused on making phone calls with Apple design sensibilities. Take an iPod Nano, insert a bigger battery & cellular chip, support Bluetooth headsets, sell millions.
Methinks coincidence though. Four icons is just enough to get the invite info across in a cute way, and there just isn't another suitable way to arrange them (a 4x1 line? not for advertising).