If self.parent is not supplied, then blizzard is adding it for itself. I looked at the code, believe me.
This has nothing to do with my findings.
Even with your code I get the same error:
Message: Interface\FrameXML\UIPanelTemplates.lua:379: attempt to perform arithmetic on field 'cursorOffset' (a nil value)
Time: 09/16/10 00:55:04
Stack: Interface\FrameXML\UIPanelTemplates.lua:379: in function `ScrollingEdit_OnUpdate'
Interface\FrameXML\UIPanelTemplates.lua:356: in function `ScrollingEdit_OnTextChanged'
[string "*:OnTextChanged"]:1: in function <[string "*:OnTextChanged"]:1>
Well the functions ScrollingEdit_OnCursorChanged, ScrollingEdit_OnTextChanged, and ScrollingEdit_OnUpdate are the same on live and beta. That leaves the parenting of your ScrollFrame/EditBox in question (since it is not shown) or you are overlooking the ScrollingEdit_OnCursorChanged difference between what I posted and what you posted (since that is where cursorOffset and cursorHeight are set).