If you go to change the default application to launch a file with a specific filename extension, say ".jpg", and you find that the "Change Application" button is disabled, you can fix this from the terminal as follows.
Note: you'll need the developer tools installed.
Let's say you have a directory of .jpg files that you want to fix.
foreach filename (*.jpg)
/Developer/Tools/SetFile -c "" $filename
This removes the "Creator" attribute from the file and this enables the "Change Application" button in the Finder's "Show Info" panel (command-I). To fix just one file use...
/Developer/Tools/SetFile -c "" filename
(Editor's note: There are also GUI apps that can delete creator codes; check macosxapps or versiontracker and search on 'creator')