Microsoft claim IE8 has a known bug that prevents Rich Text Editor from opening (see http://johnnliu.spaces.live.com/Blog/cns!90A843AB92E99F!471.entry?sa=569586369) .
Using the IEdevloper toolbar I’ve found the problem is in HTMLeditor.js file, the web part id is coming through as an empty string, when there should be something there. Now when I browse the same page in IE7 standard, I can open the rich text editor successfully.
You can get around this by setting IE8 to run in compatibility mode (essentially it emulates IE7) by clicking Tools > Compatibility View from your IE8 browser. You should then be able to open the Rich Text Editor.