Update: On October 20, 2009, Apple introduced the successor to the Mighty Mouse, the Apple Magic Mouse:
It eliminates the scroll wheel completely, so you’ll never have the scrolling problem again! The whole upper surface of the Magic Mouse is a multitouch touchpad. If you buy one through this affiliate link, I get a little percentage that helps me maintain this site and keep up on the latest technologies. Thanks! 
A few times now, the Mighty Mouse on my Mac (the one with the tiny little scroll ball on it) has lost the ability to scroll down. It scrolls upwards, and horizontally, with no problem. Seemed like a software problem to me, and in the past, restarting the app I was using, or the whole Mac, seemed to fix the problem.
It started doing it again this morning, and this time, no amount of rebooting, unplugging/replugging, etc. would work. I figured it had to be a software (or maybe electronic) problem, since if it was just gunked up and dirty, I figured it would impair scrolling in all directions. Not just one.
Well, it turns out, when the Mighty Mouse scroll wheel gets dirty, it stops being able to scroll down. Apparently, it’s a common problem, as a quick Google search turned up tons of pages, like this one at MacOSXHints.com.
The fix? Blow compressed air around the scroll ball, or just push on it really hard and roll it around (that worked for me). If you’re lucky (I was), it will start working again. If it keeps doing it, though, I’m going to have Apple replace it.
What a weird problem. Design flaw? Who knows. I guess it would be better if it were easier to remove and clean the scroll ball. Apple’s official directions for cleaning it are “hold it upside down and roll it around”. Seriously. They even have a video to show you how to do it.
TinyScreenfuls Feed (this blog)
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Yeah, pretty easy fix with compressed air, pushing (not too hard) the ball with a lit more force. It dislodges gunk. Does not matter how great a piece of gear is, it will always require a little maintenance. Mine is fine again.
Thank you so much. I can’t believe how quickly your suggestion worked!
This was great. The canned air worked immediately.
Now I can’t believe this….and it’t not because I’m tight…..but I obviously had the scrolling problem (had it for ages)…never bothered with the car of air, I just puckered up and blew in the dam thing. Oh my god it worked….scrolling now ok!!
So the whole blowing on it thing actually worked for me also!!!
Wow, I just blew a few times while pushing the scroll button and it does work! sweet. Who knows how long until I have to blow again
Thanks a lot for the posting. That was really helpful, and it definitely works.
Still I’m not too convinced. I have also this problem and so replaced the Mighty mouse by a solid Microsoft optical mouse. Guess what – the same happens from time to time: I get stuck at the bottom of a web page in Safari with no possibility to scroll up. Usually closing and reopening Safari clears the issue. It sometimes happens in other apps too. It seems there is also some software issue…
Thanks for the post, cleared the problem right up!