My Mighty Mouse decided to only scroll in one direction this week (up), so I found a thread on macosxhints.com that had some suggestions. You can't disassemble the Mighty Mouse easily, so you have to work with what you've got accessible to you from the outside.

Turning the mouse over and rolling the ball with my finger worked earlier in the week, but today that didn't help. I tried the "wet finger" routine, which also came up without any results. Finally, I took some Windex, gently sprayed a wad of it on to a paper towel, and squeezed out the tiniest drop on to the mouse ball, then rolled it around with my finger before rolling the ball upside down on a flat paper towel. After doing this a few times the scroll wheel is back to normal.

I think I needed to go a little further with the cleaning routine since I've never cleaned my Mighty Mouse in the two (maybe three?) years I've had it. I've been using it just about every day since I joined Highwinds so I'm sure the increase in use contributed to the stickiness.