Problem:
An application has turned into a plain folder and stopped working, e.g. after moving the application to another drive.
Solution:
Simply add the ".app" extension to the name of the folder (that used to be an application) in the Finder. Doing so will turn it back into a working application package -- provided that the ownership and permissions of all files in the package are still correct.
Background:
After I moved an application (Virtual PC 6, it was) to another drive, it stopped being an application and turned into a plain folder. I tried adding the "execute" permission to the whole folder, recursively, to no avail; it simply would not turn back into an application package. I looked all over the place for a solution but found none. After reading somewhere about bundles, however, I tried adding the .bundle extension to the folder (that used to be an application package). The folder turned into a blank file icon and, when I double-clicked it, I received an error. I then remembered the extension ".app" and tried it instead. Bingo! The folder immediately turned back into an application package, complete with the application's icon.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040205122223531