I've been using this layout at the office for some 3 three years now. Admittedly I use it on a Dell Laptop when connected to an external monitor, but it's really useful for me.

In my case I develop a lot of applications for the local business, most of them web-based. The fact that 90% of the users have the same laptop as me and that the monitor when rotated has the same width as the laptop makes it easier to do things that will look good when viewed in the smaller screen, while still leaving me plenty of space below for terminals and IM chats.

I liked vertical since the old radius monitors, and set-up the laptop to vertical for the external monitor as soon as I could (I should note how terrible Windows is handling multiple monitors.

I should note, too, that it seems no Windows user had ever known that could be done (both the spanning and the rotation) and I've just reinforced the idea that IT people (and macheads especially) are weird. :)