Same problem at my MacBook Unibody 13" (late '08):
every time I am connecting my external monitor, the desktop icons on the MacBook are not clickable. Disconnecting or restarting the Finder brings it back to function. Still no other solution found
I have similar symptoms; desktop items unresponsive to mouse clicks but dock still works. Relaunch of the finder works for me.
I do not have iStat installed, but I tend to see this when utilizing VNC to access my different Apples on my local LAN. If a session times out or is not disconnected, I will see this behavior.
I get the same problem on my MBP with 10.5.1. It has only happened a couple of times and I can't be sure what causes it.
To solve it, I type killall Finder into a Terminal window to relaunch Finder or you can Alt-Right Click on the Finder icon in the Dock and select Relaunch.
I had this problem too...had folders on my desktop that wouldn't open. Tried deleting them, selecting them, nothing would work. Tried the "alt + right click, select Relaunch" and it got rid of them. Thanks!