It seems the FCKeditor module has a problem when the JQuery Update module is also installed. Loading a page with the FCKeditor on it throws an uncaught exception in Firebug:
uncaught exception: Syntax error, unrecognized expression: [@class=teaser-button-wrapper]
Then clicking on the "Switch to plain text editor" button throws another:
uncaught exception: Syntax error, unrecognized expression: [@class=teaser-button]
Disabling the JQuery Update module fixes this however there are several modules I use that required the JQuery Update module.