It does spin down after about 5 minutes when the NSLU2 is switched off so I get the impression the spin down is working it’s just that there must be some sort of activity that is keeping the drive spinning all the time? Perhaps this is a side effect of not using a USB memory stick for the system drive?
You’re right, something is accessing the disk from time to time. There are various things you can do about this in a single disk set up such as mounting certain areas of the filesystem (e.g. /var) in a ramdisk or mounting things with -noatime. You’ll need to pick through the Unslung wiki but all the info is on there. Here’s a page that might help:
The starting point would be to figure out what’s doing the disk access though…