So for about the past year the clickwheel on my 40G 4th gen iPod has been slowly dying. It started with not being able to scroll with about the top quarter of the wheel. After a while of this, the scrolling stopped working completely.
The cheapest replacement I found was from RapidRepair (http://www.rapidrepair.com/shop/1409-clickwheel-4g.html) for about $10. I then followed this video
http://www.ifixit.com/Guide/Replacing+iPod+4th+Generation+or+Photo+Rear+Panel/753/1 to remove your rear panel to access the iPod’s internals and this video on Vimeo
http://vimeo.com/1669495 to change 4th Gen iPod Click Wheel….and my old venerable iPod is alive again…if a bit finicky at times.