Discovered this last week.
The first time browser access a page with RadSplitter(s) in it, the element RAD_SPLITTER_PANE_CONTENT_xxxxx_uxRadPane does not have CSS width property. When the user issued a post back the CSS width property back to RAD_SPLITTER_PANE_CONTENT_xxxxx_uxRadPane.
This behaviour does not cause any layout problem in left to right layout but will sure affect right to left layout. In my project right to left layout is totally displaced and I have to manually use JavaScript to issue a form post back to hack it right.
By the way this issue/bug seems only affect IE. Other browsers are fine with or without JavaScript hack.