Are there headset adapters usable for MacBook Pros?
Of course the combined jack on Macbook is analog, of course it accepts mic signals (works with apple and other brand headsets for iPhone, for example) and not just line in signals (you can even plug electric guitar with passive adaptor from iRig) and as Zneak properly points in his question, it would not be too difficult to make an adaptor that splits one headset jack on Macbooks into headphone and mic, which is also great if you want to use some top notch headset like Sennheiser PC 360. Unfortunately I was not able to find such a product, only a guide on how to make one.

Last but not least, USB headsets are in no way better. They only use extra CPU power, clutter USB ports and are only as good as the DACs, ADCs, headphone and mic they employ. Now these components are much better inside Macbook and good analogue headset...



The Headset Buddy at Amazon claims to do just this. But I am not sure how well it will work, as the headset pinout Apple uses in Macbook, iPhone or iPad is different from some other manufacturers (that is why you cannot use iPhone headset with, say Samsung Galaxy S)