I have a view which displays different taxonomy terms, I would like to set a different "header text" depending on what term the view is filtering by. Right now whatever header text I choose I can only apply to all pages of the view. I would like to display different header text depending on what term is being filtered.
Instead of a Global: text area, you should embed another (new) view in the header.
This blog post explains how you can display the term description that way. That way you would have a different text for each term.