This is not a simple process. The button itself owes a lot to the technologies in Mac OS X, so I don't think it was (or could be) directly "created" in a drawing program. But you might be able to simulate it in Illustrator 9 using the transparencies and layer modes:
1. Draw a button shape, fill with color. Use soft gradients to round the button form, set with some transparency and layer modes for translucency.
2. Since translucency is involved, put a background on a layer below. Rasterize it for future steps (optional at this stage).
3. Above these layers, type some text, with transparency about 75% to keep the button translucent.
4. Duplicate the text layer, lower it and rasterize it as a layer. Gaussian blur to soften shadow. Set transparency to less than 100 (about 40%?) and Multiply into bottom layer (background)
5. Copy the button shape, fill with black, rasterize, Gaussian blur, and set to less than 100% transparency. Move layer above other bitmap layers and multiply.
6. Merge the two shadow layers, and background if you don't need to change it.
Once again, this won't be exact, because the "Quartz technology" of OS X isn't in there. And you won't be able to place this over a background and have live shadow effects. But you could put this over a matching background (at 100%) and have a good impression.
Some third-party plug-ins (KPT or Extensis, for example) might speed up the process of modeling the button and setting appropriate gradients. And you might, if you prefer, build the soft shadows with gradients as well, but the overlay aspect could be a bit more complicated.