IDL has never had a single consistent style. There are many styles, none of which I liked (mostly my aversion to ALL CAPS and lack of indentation). I created a style that I would be happy with. I recently uploaded it to the [developer's page](http://idldoc.idldev.com/wiki/DeveloperInfo) on IDLdoc project site.
I did not write this to tell IDL programmers what style to write their programs. If you already have a style, then no need to read any further. But if you're looking for a style for your group/team, then check out the style guide. If you don't like the choices it makes, it can at least be used as a template for specifying your own choices.
Many programmers new to IDL simply aren't given any guidance on what style to use. The IDL library, examples, and documentation use various styles. Third party groups have their own styles. For a lot of programmers, it doesn't matter, they just don't want to have to agonize over every small formatting and naming decision. But they do want a well-defined style for everyone on their team to follow.