strange problem, when I use Director::addRules(50, array( ASSETS_DIR."/$Action" => "SomeActionController")); I get the notice: "Variable Action not defined.". However, I don't see how other _config.php files do define this 'variable'. Something I'm missing?
This is because you're using double quotes rather than single quotes. With double quotes, PHP tries variable substitution on the string. With single quotes, it leaves the string as is. (see [url]http://nz.php.net/manual/en/language.types.string.php#language.types.string.syntax.single[/url]).
Using Director::addRules(50, array( ASSETS_DIR.'/$Action' => "SomeActionController")); will work fine.