The latest issue of A List Apart deals with having pullouts span multiple columns.
For the uninitiated, a pullout is the name for the box above that says “Can this box span multiple comuns?”. Floating it within a single column of text is pretty simple. Spanning more than one column is a bit trickier.
It looks great on a printed page, but I’m not sure why you’d want to create a multi-column content layout for screen output anyway. Why force your readers to scroll up and down the page more than once? I think the practical applications of this one are fairly limited. Although, having said that – I’m sure it could form the basis for something more practical.