Use Macbook Pro's single audio jack for both input and output at the same time?
Yes - the good splitters mix the pins correctly to provide mono microphone and stereo output in the same jack. You can of course get breakout boxes like the Griffin iMic and have the benefit of a cleaner D/A circuit at the expense of carrying another device that's plugged into USB. Just like everything - some of the expensive products are over priced and some of the cheap ones are worthless. I'd go with a name brand - especially if their products are carried on the Apple Online or stocked in the retail stores.

Lion does add a control where you can select the speaker icon in the menu and switch from input and output.



As to the rationale - unless an engineer from Apple steps up, we'll have to speculate.

Here are the reasons to have fewer ports. (They're not al good, but they are reasons)


Simplicity - the iPhone / iPad / iPod touch all have one port. Many people don't need or want the confusion of knowing which port they plugged in to their mac. Just as you really appreciate having two, many appreciate having only one stereo jack.
Internal simplicity - one less switch, one less moving part, several solder points.
Internal spacing is far more important than external space. If you look at iFixit's excellent take apart manual - you will see there actually isn't room on the logic board for another connector. The design would have to sacrifice an USB port or the SD card slot to make room for a second package without reducing the space for the battery. These macs are packed incredibly tightly and despite the clean appearance of excess external space, inside they are packed like proverbial sardines in a can.


This lack of space likely cuts features from the smaller macs like a redundant mini headphone port.