i have button 3 set to Exposé all windows, button 4 set to Spaces and just hit F12 for Dashboard.
i love having Exposé set for the scrollball. when im using Windows i always find myself pressing the scroller and expect all the windows to go flying out 😀
Back before I got my Mighty Mouse, I was using a 6-button rodent, and 4 was set to Expose - show desktop, 5 to Expose all windows, 6 to Expose - application windows.
Logitech MX Revolution. I consider five buttons the minimum acceptable on Windows, to have forward / backward buttons. And with Apple's Expose and Dashboard features, I think that contrary to Apple's aesthetics, even more buttons are better.
And unfortunately the Apple mouse is only a four button mouse (left, right, ball, squeeze), so it's not a good solution for me.