First off, thanks for releasing Monkey Island for the Mac!
I use a non-administrator account on my Mac. Normally, when I need to install an application to my /Applications folder, Mac OS X prompts me for the username and password of a user with administrator privileges. Successfully providing these credentials results in the application being copied over. After the application has been copied, I can launch and run the application without any further authentication or issues.
Dragging Launch of The Screaming Narwhal into my /Applications folder prompts me for the username and password of an administrative user. After I supply these credentials, the game is copied into my /Applications folder. However, when I go to my /Applications folder, the game is displayed as a folder with a red minus icon in the lower-right (see attached screenshot Finder.png). Attempting to launch the game results in a popup window with the message: "You can't open the application MonkeyIsland101 because it may be damaged or incomplete." (see attached screenshot Snap.png)
Lastly, I'll add that copying the game into a folder for which I don't require elevated privileges works correctly. This is currently how I have Monkey Island installed so I can play.