I've tried to edit a field in a few content types to set the filtered text format to something other than filtered html, but after I save, it has no effect. I go back to the content type, and it's back to filtered HMTL. If I select "Plain Text" form the "Text processing" setting, I can pick plain text in the "Text format" dropdown, but otherwise, it always saves back to "Filtered HTML". What am I doing wrong?
As far as I know, this functionality is "broken" in D7, meaning that the UI is at least misleading. Here's some discussion https://drupal.org/node/1003262
What you can do is change the order of text formats at admin/config/content/formats. The default text format available to users will be according to permissions and the order of text formats.
Also see http://drupal.org/project/better_formats. That will give you more options.