Where the hell is the Launch Calculator entry in gnome-keybinding-properties ?

Short answer: There is no entry like in Ubuntu. I searched the web, but nobody knows why. The workaround is to create a custom shortcut.

HOWTO:

• gconftool-2 --set --type string /desktop/gnome/keybindings/calc/binding "XF86Calculator"
• gconftool-2 --set --type string /desktop/gnome/keybindings/calc/action "gcalctool"

replace calc with something that describes your shortcut.
replace XF86Calculator with your key, use the command xev to get the name of the key
action is the executed command when the key is pressed.
Restart your Gnome session. I think logout and login will do.