Has anyone seen this before? I can't find it in the docs.
Serial.println( F("What is this for?") );
I've seen it recently used by some people, I know what it's purpose is but haven't used it myself. It's to force the compiler to store the constant string into Flash memory of the AVR chip rather then the limited SRAM memory where normally all variables, constants, and arrays are stored. A 328P chip has 32KB of flash memory but just 2K of SRAM space, so you can see that most people tend to run out of SRAM before they run out of FLASH in their larger sketches.