If you try to open a file in the Finder and it doesn't recognize the extension, it will now offer to search the Mac App Store for applications that can open that filetype. Before the Mac App Store's launch, the alert panel you see above would simply say, "There is no default application specified to open the document 'foo.bar' " and offered only "Cancel" or "Choose Application" as choices.
Ideally I wish the Finder would tell you if you already have any applications installed which could open those files, but this is a nice addition for people who might receive an unknown file and not know what to do with it.
Finder now offers to search Mac App Store for
