UPDATE: The shortcut in Lightroom 6 and CC 2015 is just F.
While I know a fair amount about Lightroom, I am always picking up more from my fellow bloggers. Here’s a quick but useful shortcut from Sean McCormack over at Lightroom-Blog.com: To see your image and nothing but your image, type Shift-Ctl-F on the PC, or Shift-Cmd-F on the Mac. This will hide the surrounding panels, menu bars, tool bars and system task bars, and your image will be displayed as close to full-screen-size as possible . While in this view, you can use your left and right arrow keys to scroll to other images. This shortcut works in all modules, and your other module shortcut keys will continue to work, such as 0-5 for stars and P for Pick/X for Reject in the Library module. Hit Shift-Ctl/Cmd-F again to exit this mode.
If you are going to be viewing your images full size on the screen like this, you should set your preferences so that your standard-size image previews are screen-size. Go to Lightroom (Mac) or Edit (PC) > Catalog Settings > File Handling, and set the Standard Preview Size to be as close as possible to the width or your monitor’s resolution. Check your system display properties to find out what this is. (Update: in Lightroom 6 / CC 2015 set it to “Auto” and Lightroom will detect the size of your monitor.)