Unfortunately, the scroll pea on the Mighty Mouse is very prone to clogging. If it's crumbs (and I do like to eat at my computer), turn the mouse over, press in the pea and roll it. That should dislodge any bits and allow them to drop back out.
The other problem is the oils on your hand will eventually cause the mouse not to scroll. I have had my Mighty Mouse about two months and have already had to clean it twice. Put some alcohol on a soft lint-free cloth (don't use a paper towel) and clean the scroll pea, rolling it around to get the alcohol down inside to dissolve the oils and gunk.
My Mighty Mouse came with my iMac, and I am completely unimpressed with it for a number of reasons. The scroll pea is so sensitive to dirt and the oils on your hand, that I am waiting until it quits permanently. I wonder if Apple is getting a lot of returns on them.