Use Macbook Pro's single audio jack for both input and output at the same time?

Question

Basically, the Macbook Pro 13inch (early 2011) has only one audio jack, but I need two.

What I want to do is to plug in my electric piano (I have a male-to-male audio cable, I plug one end into the piano, the other in the microphone jack) into the computer, then play some backing track on the computer, and finally plug in headphones into the computer so that I can listen both to what I'm playing and the backing track.

This worked perfectly fine when I used my netbook. However, now I would like to use my macbook, but it has only one jack, so I can't plug both the piano and the headphones in.

Is there some kind of a splitter I could use so that I can plug in both at the same time? I found some threads online, such as this one, but some of the posts seem contradictory, so I thought I'd post here.

I would also welcome any answers that could explain the rationale Apple used for not including a second jack. Please do not use size as an argument; 10 inch netbooks fit 2 jacks just fine.

Thanks!