I got my original one replaced after it too stopped scrolling. My second one I decided to really look at ways to clean it so it will work again and I have found that if you spend a few minutes (and I mean minutes not seconds) with a damp cloth (after unplugging the mouse of course) and scrolling the ball in all directions and occasionally holding the mouse upside down to free any trapped particles it will eventually work perfectly again.
My exact process is unplug the mouse get a damp lint free cloth and work the scroll ball for a minute or two then check to see if there are any particles of crap on the ball. Move the ball around slowly because sometimes bits on the inside will stick to the ball and because they are damp will come around to the top so you can get rid of them. Then go at it again with the damp cloth. Keep doing this on and off for a good few minutes occasionally turning the mouse upside down as you do it. Once you think it might be clean let it dry (or dry the ball with a dry lint free cloth) and attempt to scroll around with it. If it doesnt work simply keep going at it and it WILL eventually come clean and work.
I was so frsutrated with mine once I pressed the ball down so hard I thought I had actually 'smoothed' out the bits inside and that it would never scroll again. Then (months later) I decided to see if I could get it to work again and I did the above tips and it works like it was brand new.
So in essence even the most knackered of mice can be cured but you have to work at it. I did mine while watching TV so I wasn't rushing to get it back in the computer and zapping aliens again.
EDIT: If the ball is still wet it will feel like it hasn't worked. Let it dry before you try scrolling.