This is not a helpful response, but it is what I did after having 3 consecutive Mighty Mice with the scroll wheel problem:
If it is still under warranty, call AppleCare, get them to ship you a replacement...and when you receive it, sell the Mighty Mouse on eBay and use the $ you receive to buy a mouse that will actually work.
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I live in a town where one of the three or four Applecare call centres in the world are -- many of my friends work there, and they say that the one thing which occupies more of their time than anything else is the constant replacement of broken Mighty Mice. They are simply a pathetically bad product.