Also appears in several other Heinlein novels, including "The Moon is a Harsh Mistress," where all restaurants served the meal free, but charged heftily for beverages, etc.
More widely used around the time the book was first published, but can still be found circulating in dark basement apartments and gaming stores everywhere.
"I answered that ad for a free RAZR and now all I get in my inbox is spam. TNSTAAFL strikes again."
"That new pizza joint totally TNSTAAFLed us."