Yesterday I was in a presentation and because we wanted to see most of the screen, there was a requirement to hide the top ribbon see the image below.
I suggested to double click on the menu item to hide the ribbon. That is, double click either on Home, Insert, Pge Layout etc.
Someone in the room suggested a keyboard shortcut Ctrl+F1. That worked too.
Also keep in mind that this works across all Office 2007 and 2010 applications including Excel, Powerpoint, Access, Outlook etc.
I thought of sharing this here so that in case you have a similar requirement you could use it.