If I remember correctly, in ArcView 3, after a scale bar had been created, you could change its properties so that it would show the scale for a different data frame than the one that was used to create it. I think the same applied to north arrows.
In ArcGIS 10, when you create a scale bar, it displays the scale of the data frame that is active when you create it. Is it possible to change that association after the scale bar has been created? As far as I know, if you want to use that scale bar (and whatever customizations you've made to it) with a different data frame, you can't, you need to delete it and insert a new one, re-creating any modifications you've made to the scale bar.
I'm asking because I have an MXD which has a layout I like, and I want to paste a data frame (containing a map I need) into it. But I can't re-use the scale bars and north arrow which already exist in the MXD, I'll need to recreate them.