Audio input/output splitter for MacBooks without changeable input/output jack? From what I can understand, in the latest iterations of the MacBooks, what former was an input/output 3.5mm jack, now only can be used for input. However, it's possible to use certain headsets (for example the one that comes as standard with the iPhone) and get both audio in and out.

Shouldn't it be possible to build an audio input/output splitter for the mini jack? That is, a cord that splits up into an input and an output part, resulting in two female 3.5 connectors, to which you can connect your headphones and your mic? Is there even such a product out there?
Yes, they are called 3.5mm 4 pin splitter, they separate a 4pin 3.5mm jack in 2 standard 3.5mm jack (1 for headphones, 1 for mic) as this one:

Startech.com 3.5mm 4 Pin to 2x 3 Pin 3.5mm Headset Splitter Adapter - M/F

you can read the comments, there are also some mac users that bought this splitter.