From AppleScripts by Tom X:
iBackground Scripts - Simply drag and drop an AppleScript that has been saved as an 'Application bundle' on to this droplet. Clicking on the 'Yes' button will set your script to run in the background and will not display in the Dock. You can reverse it by clicking the 'No' button and it will no longer be a background script and will display in the Dock. If you don't see the change immediately, move the AppleScript to another location (Command+drag). Note: When saving a script as an 'Application' on Snow Leopard it is in fact an 'Application bundle'.