It's not that hard to take apart a mighty mouse and clean out the scroll ball mechanism. I did it recently with a three year old wired mighty mouse to see if I could do it - it was in a drawer and not being used. I did break a couple of things that I'll mention later.
Anyway, I didn't take any pictures, but you can see a disassembled mighty mouse here -
http://theory.isthereason.com/?p=539#comment-11703
You open it by prying it apart at the side buttons. Stick something pointed like a letter opener and pry the top and bottom apart. This is where I broke the first thing - the ring of plastic that runs around the base and under the side buttons seems to be attached to the top - a bit of super glue should fix this, but it doesn't seem to have any function so I didn't bother yet. I think, but I'm not sure, that you should pull off the side buttons first if you don't want to break the ring.
The top will then be loose but it's still attached to the base by two plastic prongs. It's easy to pull them apart. This is where I broke the second thing. The top is attached to the circuitry in the base by two ribbon cables. I didn't realize this and tore one of them. If you're more careful than me you can just pull the ends out of the sockets in the base. It turns out that the ribbon I tore controls the function of the left, right and middle button press. I now have a one button mouse that thinks the middle button is being pressed, but I managed to change the mouse settings so that the middle button is mapped to the primary key and it's just like a standard one button mouse.
Look again at the picture in the web site I linked to. The scroll ball mechanism is in the black box attached to the brown ribbon. You can pull the white cover off and you'll see four little axles each with a black wheels like in this picture -
http://sucs.org/~dez/blogfiles/mightymouse.jpg - mine weren't that dirty but had a lot of gunk on them.
Clean these up, and reassemble the black box. Make sure the black wheels are lined up with the metal plates in the box - they are magnetic and the mouse seems to use induction to determine the direction of motion. Push the white cover back on with the ball inside. Scroll the ball in each direction and look closely to make sure each of the little wheels is moving freely.
Reassemble the mouse and you'll have one that's as good as new, as long as you don't tear the ribbon cables. If you tear the green one, you'll still have a functioning one button mouse with scroll, but if you tear the brown one, scroll won't work at all.
