I have the Apple Optical USB Mouse and a iMac C2D. The little button that allows horizontal/vertical scrolling has failed a couple of times in the month and a half that I've had this mac.
When it failed it was only the scrolling back up to the top of the page that wouldn't work. I fixed it twice by going into System Prefs and fiddling with the Scrolling and Tracking settings.
I've seen other threads on the Mac-Forums where people air their feelings about their failed mice so I guess I knew I would experience problems sooner rather than later.
Recently the upward scrolling has failed and I can't fix using the above method so while on one hand I'm thinking that Apple should take responsibility for this dodgy scroll button and either I'd take it into the AppleStore and ask them to show me that it's possible to have the button functioning again with a little house-keeping, or Apple would replace it with a new mouse.
Then on the other hand I'm thinking that I should just buy another mouse which I know would offer more reliability.
In actual fact, Im so used to the older mice which didn't have the scroll button that I almost don't really mind using the scroll bar in the active window/browser, but I guess I'm going to have to look into the track ball cleaning option before I do anything else.
You might find this link helpful but take care and read everything carefully as you are following this advice at your own risk:
http://repair4mouse.org/disassembly_apple.html
UPDATE:
I just fixed the scrolling by having a Safari window open and then pressing and holding Control and rolling the scoll button. While this zooming was still working anyway, doing it then letting go of Control somehow got the scroll button to correctly scroll both upwards and downwards....so it wasn't an issue of dirt at all in my case, more like a bug.