I ran across the expression "as useful as a chocolate teapot" (or sometimes a fireguard) which is apparently used to denote the utter uselessness of something. It received some coverage on Language Log back in 2004. Where/how did the expression originate?
Eric Partridge and Paul Beale's Dictionary of Slang and Unconventional English (8th edition, 1984, s.v. much use as) gives an example of "chocolate teapot" from the Guardian, January 1979.
I haven't found any earlier examples, and don't know where it came about; though there are a few references to it on lists of Irish slang, I don't personally associate it with Irish English. The logic is presumably that if you used a chocolate teapot then it would melt, though research shows that this is not always true!
The expression was used in The Guardian of 17th July 1978 in an article titled "Barnsley bashers face the chop", written by Michael Parkin:
Tourists that go to home games of Barnsley Town will hear some of the finest football wit and repartee in the land. Players are accused of being "as nimble as a stone trough" or "as much use as a chocolate teapot."
The phrase is repeated in The Guardian of 13th February 1980 in "Barnsley go down and the fans love it", again by Parkin:
What the visit will do for the public relations on the terraces at the Oakwell ground has yet to be seen. Barnsley's supporters are notoriously critical and much given to witty judgments.
A visit down the pit will not save a dawdling player form the kind of remark in which the Oakwell crowd specialises: "That's as much use as a chocolate teapot."
A 1981 Google Books snippet also points to the terraces as origin for this ironic simile. Desmond Morris's The Soccer Tribe (1981) describes 'The Jokers':
If the referee gives the opponents a penalty, he yells, 'When they circumcised you they threw away the wrong part.' If a player is not trying hard enough, he bellows, 'You are as much use as a chocolate teapot.'
Wikipedia confirms the date and says:
In 1978 Desmond was elected Vice-Chairman of the Oxford United Football Club. Interesting to note, later in 1981 Morris publishes 'The Soccer Tribe,' analysis regarding the world of professional football, or rather soccer, to Canadians and Americans.
I found this in a Google Books snippet (so no further context) from The Accountant for 1981:
... is managing director of a hardware firm, singing IF's praises — in the hope, no doubt, that others in the profession will see the advantages and not consider, as does the Cleese-consultant, that 'it is about as much use as a chocolate teapot'.
This suggests that the phrase may come from one of the many humorous business training videos which Cleese's company Video Arts produced in the 1970s.
'Michael Parkin' as referred to in Hugo's answer is the British journalist, author and chat show host Michael Parkinson. 'Chocolate teapot' features in his book Football Daft, published 1968.
In that book, Parkinson, famously from the Yorkshire town of Barnsley, describes Tottenham Hotspur coming to play Barnsley FC at the latter's Oakwell ground, a game at which Spurs and England fullback Alf Ramsey was tormented by a tricky winger playing for Barnsley. Parkinson says that a wag in the crowd at one point shouted "Ramsey th'art about as much use as a chocolate teapot!" and further posits (for comedic value) that this incident was the root cause of Ramsey's later aversion to wingers, as seen by his team of 'wingless wonders' that won the World Cup for England in 1966. The fact that he used the phrase later in other contexts suggests to me that it was of his own invention.