I had the same problem with the first 5th. gen I took apart. My comments on the spring went unanswered too. Came to my own conclusions.
First the odd looking bit of black plastic (which usually flies out when you pry the click wheel assembly out). If you look at the main board you will see slot about 5mm long by about 1.5mm. Take the black bit in some forceps and you will find it will click back into this slot. There is a hole in the black bit which holds the tiny spring. If you look at the ipod case, preferably with everything removed), there is a small patch of silver plating on the colored anodized surface. My theory is that the spring is a grounding contact between the case and the center of the click wheel assembly. That spring is a little devil. If you drop it on your workbench, even on a white cloth, its so small - just blink and its gone! The only way is to open up carefully then pickup the spring with a piece of blue tack or Scotch tape - and leave it there until you need it. Most of my springs have disappeared and are never seen again. However all the i Pods I've worked on seem to work OK without them.
It's just the thought of that spring rattling about inside the circuitry that makes you feel uneasy. I suspect they end up being stuck to the speaker. I've never found one yet!