It should be compatible with IE8 since it is the latest release of FCKeditor... but I know from experience, that FCKeditor's latest version still has issues on Apple Safari, regardless of the Operating System, so even with Windows 7, Safari may experience issues with FCKeditor.
The other issue I have seen on Safari is that objects (such as images) are not selectable in the edit window, so you cannot resize images on the fly in the editor. I have had this problem with not only FCKeditor but also other editors such as TinyMCE and the newest CKeditor.
As far as if there will be any issues with Windows 7, I believe WA has stated that since Windows 7 has not been released, no testing has been done. But they have stated they would be quickly issuing compatibility releases as needed.