Here's a bit of an obscure hint to change which SubEthaEdit "modes" respond to which extensions. I sometimes use php code inside of .html files (rather than use the files with the .php extension; don't ask why), and I use SubEthaEdit to edit the .html files. I wanted to make SubEthaEdit open .html files with the PHP-HTML mode instead of HTML. Here's how...
In the Finder, control-click on SubEthaEdit and select Show Package Contents. Then navigate to Contents -> Resources -> Modes. Select the mode you want to alter, and again control-click and select Show Package Contents. Then open the Info.plist file inside the package.
Alter the TCMModeExtensions property to your liking. Be sure to change the HTML mode appropriately to avoid undefined behavior. Hope this helps someone.
[robg adds: I haven't tested this one, and you'll probably want to backup your SubEthaEdit app before you modify its contents.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040608220938410