OK, so Apple has finally released a multiple-button mouse. It's even (finally) got a scrolling mechanism; something that I think may have been a major driving force behind its release. However, it still only has one main button, & there might just be a reason for that....

"Apple supports multi-button mice. Right out of the box. Furthermore, this is not a ‘new feature’ of OS X. Support for contextual menus (the primary use for the two button mouse) have been around since OS 8.6, which is now more than seven years old."