Our brand-new BarredText plug-in allows you to anchor a vertical bar into the margin of selected text (for example, to visually mark edits to a text). Once the bar is anchored, it remains attached and will automatically track changes to the text. It will move, expand or shrink as needed when text is added to or removed from the marked area. BarredText supports InDesign CS3, CS4 or CS5 and sells for US$19.00 for a single-install license.
More info here: