It's more than simply shorter. The diameter of the center plug is also one mm smaller.

In a pinch, you can use a simple 3.5mm-to-2.5mm stereo adapter to power up a newer 'book with an older yoyo. Radio Shack and others sell these very cheaply (a couple of dollars US). Just get one with a rubbery plastic molding, and cut it back enough to allow proper insertion into the larger the 3.5mm yoyo plug. The result is not mechanically stable or pretty, but in an emergency, it gets the job done.