The new keyboard found on MacBooks and MacBook Pros now contain Dashboard, Expos??, and media controls on the function keys, but removed the toggle between mirror mode and extended desktop mode. On the prior keyboards, the toggle display mode function was located on the F7 key.
On the new keyboards, you toggle display modes by pressing Command-F1. This only seems to work from the built-in laptop keyboard.
[robg adds: From the "coming full circle" department, this behavior existed way back in 2002, then apparently was either forgotten about or disabled on newer laptops, only to make a return on the new models. I'm running this one now because the keyboard layout on the new machines lacks the mirror function on F7, so you may not think to try (nor perhaps be able to find) a 2002 hint as the solution.]
