Gecko support for CSS3 multicolumn layouts looks interesting... I know next to nothing about CSS3 in Mozilla, but this is something that I have wanted to use on a number of occasions.
To create a column layout, you would just need to know the number of columns, and the width and the height of each column, and the script would thus be able to re-calculate the flow of content. It would be nice to control as much of the layout as possible through CSS, but I'm still a little uncertain about what the overlap between style objects and computed styles should be.
The code for placing arbitrary paragraph breaks would be interesting. May require some kind of string parser or backtracking regex. I'm tempted to bend in the direction of treating the document as template strings, rather than trying to rely on verbose DOM maniuplations, although there may be something in there that would be helpful. The best direction I've seen thus far is the 13th Column Script.