A patent application has been uncovered that shows how Apple plans to deliver even thinner devices by using an audio port that is actually smaller in diameter than the jack that plugs in.
The Register points to one document published last week by the US Patent and Trademark Office called "Low Profile Plug Receptacle" shows three possible versions an open socket, one covered by an extendable semi-flexible material and the third one a "hinged housing".
The iPod Touch, The Reg notes, is only 7.2mm thick while the iPod Shuffle and the Nano are around 5mm thick when the clips are excluded. As for the iPhone 4, it is just under 10mm and the iPad 2 is even thinner at 8.8mm.
It does look unlikely though that the next iPhone 5 will be less than a 3.5mm thick jack given the fact that there are other factors to be accounted for like the battery size, the screen etc.
However, we wouldn't bet against Apple launching an ultra thin iPod Nano later this year, one which could well be thinner than an earphone plug.