A caricatured staple on TV police procedurals, the Miranda warning has been gradually stripped of its original intentions of protecting naïve suspects and turned into yet another tool of law enforcement.
A ruling by the 9th Circuit Court of Appeals last week went against that dominant trend. The government has a "heavy burden" to show that a suspect made a clear and unambiguous waiver of his Miranda rights prior to police questioning, the court held in United States v. Jose Rodriguez.
"I'm good for tonight" is too ambiguous of a statement to count as a waiver.
The importance of this case is discussed in detail by Steven Kalar at the 9th Circuit Blog. For more in-depth discussions of Miranda in contemporary police practice, see Richard Leo's Police Interrogation and American Justice and Charles Weisselberg's Mourning Miranda.