I created a test page and it happens in IE8. It happens on RadDatePicker as well.
It does not occur on RadEditor.
To reproduce the issue: Type in some characters, then TAB off. then use the mouse to try to highlight the whole text from the end of the text. It'll get focus in the textbox but will not highlight. You have to perform the operation the 2nd time for it to highlight.
If you tried to highlight by clicking the very beginning of the text then whole text gets highlighted.
I just tried it with 2009.3.1503.35 and the problem no longer exists in this version!