Saying you're going to 'read the riot act' to someone is reasonably common in modern British English; it means you're going to tell them off for having done something in the past, or warn them about the dire consequences that will follow if they do something in the future. Riot act is usually written with a small r and a in this sense.
Today is the 90th anniversary of the last reading of the actual Riot Act (capital letters), a piece of legislation passed in 1715. Workers in Glasgow went on strike in January 1919 (story here) and the government over-reacted (this was shortly after the end of the First World War and soon after the Russian Revolution, and the government feared worker unrest). The Riot Act said that 'if 12 or more persons unlawfully or riotously assembled and refused to disperse within an hour after the reading of a specified portion of the Act by a competent authority, they should be considered as felons'. This could, in theory, have been punishable by the death penalty.
To hear BBC newsreader Peter Donaldson read the Riot Act, and to hear an interview with a historian on the Act, click on the 08.56 interview here (probably only available for a few more days to those in the UK as it is a BBC interview which took place on Saturday 31 January 2009).