Interesting stuff on cricinfo:
10.50am "Here's a theoretical question for you," says Chris. "If we get a washout today, and it seems only Saturday and maybe Sunday can hold play, is there scope for the captains to change it to an 'unnoficial' one-day match or 100 over a side match. Seems a little poitless watching one team bat for a day before announcing a draw? Has it happenned before?"
Well, that is basically how the first ODI came about in 1971. When the first three days of the third Test at the MCG were washed out officials decided to abandon the match and play a one-off one day game consisting of 40 eight-ball overs per side. Australia won the game by 5 wickets. Also on England's 1989-90 tour of West Indies that the Guyana Test was washed out an an ODI played on the scheduled final day. It should be added, though, that in those days it was simpler to slot in a one-dayer because they didn't use white balls, coloured clothes and black sightscreens.