when dragging / dropping highlighted text between textboxes browsers behave differently about whether the value is copy/pasted or cut/pasted between input elements -> that's not the issue.

The real issue is, that after the drag/drop the textboxes which are updated (whichever the browser decides) should send and onChange event to the server, otherwise the client/server state gets out of synch.

http://screencast.com/t/qmCjgXMBrw
(unfortunately Jing prevented the drag'n'drop in IE but can be easily tested using the attachment)