From MacWorld:
No doubt about it — Apple’s announcement that it’s selling a four-button, scroll-wheel-bearing USB mouse is big news. Not in the earth-shattering kind of way that you’d greet the announcement of a new Mac or iPod. It is, after all, just a mouse.
But the Mighty Mouse marks the end of 21 years of Apple’s one-button mouse policy. In that way, it’s a little bit like the Berlin Wall coming down. Except, y’know, with mice.