http://brooksreview.net/2011/09/tablet-power/ Ben has a really good take on the way that the iPad is giving folks more choice in the matter of display size and orientation. I really like to use my iPad in portrait (veritcal) mode for reading and navigating text heavy websites. I flip 'er sideways when viewing sites that are photo-focused or when I need to see the whole page for layout reasons.
I have a few apps on the Mac that are annoying to me on the MBP because the natural workflow would be so much better with more vertical space. I'm thinking that one external display in a vertical mode and the built in display in Horizontal would be a good setup.