Hi Kenn,
I tested FCKEditor on both 6.4.0P2 and P3 code just now in IE8, FF3.5, Chrome and Safari and had no problems.
I did follow the procedure I told you about in this thread here, to get the resources for FCKEditor, and I used the resources from the current 6.4.0P3 code you can get from SVN.
I did notice one difference between 6.3.x and 6.4.x though. The FCKEditor BasePath property need to be differently set in the two versions. There is one more "../" required for 6.4 than there is for 6.3.
Please let me know if this doesn't work for you.
Palli