They've actually supported two-button mice since OS X first came out (and maybe before). On the newest Powerbooks and Macbooks, you can do a two-finger-on-the-trackpad-and-click to right click (previously it was ctrl-click). The mice that come with the desktops have two buttons, it just doesn't look that way (it senses pressure on each side of the mouse), as well as a "squeeze" you can set for whatever.