indicate that something in your profile is not good.
Try going to your Home folder > Library > Preferences and find com.apple.finder.plist
Drag it to the desktop and restart the iMac (note, not all caps)
See what happens
P
I am running 10.6.6 on an iMAC and I can not move the desktop icons. When I try a circle with a line through it appears. I have the Finder view as "arrange by "none"", I can re-arrange if sorted by name, size, etc. but it will still not let me freely move the icons. This all just happened yesterday after working in iPhoto. Have created a new user and all works there.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic