not a great hint but useful nonetheless, the show info menu in Get Info (command-I) now allows you to change more than one file at a time and if you wish to change the 'open with app' type it gives you a list of all possible apps, so you dont have to go find the right one. you can also change all 'other' types too, so all your .img files can be set to open with diskcopy or .sit files with stuffit.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011003100216832