Removing Leopard download quarantine
MacOSX Leopard tags files downloaded from the web with a special metadata bit. This bit is later referenced to warn about running downloaded apps and scripts. Even archived files maintain this tag through the compression/decompress process.
I needed to update the firmware on my Canon PowerShot SD1000 camera and couldn’t get the camera to recognize the file. Turns out the com.apple.quarantine xattribute was set. To fix:
xattr -d com.apple.quarantine *.FIR
xattr does not have a man entry. Use:
xattr -h
I found most of my information on this helpful post.

Were you installing chdk? Was thinking of trying it out on my PowerShot too. How did it go?
http://chdk.wikia.com/wiki/Main_Page