Once again, shame on your Mighty Mouse. The scroll ball is such an intolerable piece of shit that it requires almost-daily maintenance -- in fact, run a google search for mighty mouse scroll ball and all you get is forum after forum of people saying, "I can't believe I keep buying this shitty piece of shit because it's the only thing that scrolls the way I need -- briefly."
So one of the common solutions for a jammed/dysfunctional scroll ball is to flip yer mouse upside down and roll it around vigorously on a sheet of white paper.
Apparently if you do that too many times, however, it will jam the scroll ball in, and then BAM, every click is a center click and your mouse is toast.
And can you unjam the ball? NO! Why? Because Apple doesn't give you a way to OPEN THE F'ING MOUSE. You also can't get under the ball and pull it up because, well, it's a slippery little bugger. Basically, in the process of my daily make-my-stupid-mouse-work routine, I managed to break it in some new, unrecoverable way.
Screw you guys, but bless your 12 month warranty, and failing that, bless your store clerk who will, whether he likes it or not, give me a new one for free under the duress of Furiously Disappointed Customer.