Janet Daley is quickly out of the blocks with an argument I expect many people will make, even though it is obviously incorrect.
What the "alternative vote" would mean is that it would be virtually impossible for the Conservatives ever to regain power. Labour and LibDem supporters could simply institutionalise their game of tactical voting: by always placing each other's party as their second choice, they would guarantee Left-liberal coalition governments forever.
Unless Janet thinks that Ken Livingstone is still Mayor of London - and that Boris Johnson lost.
In fact, under the supplementary vote system (a simpler cousin of AV), Ken Livingstone received 135,000 second preferences and Boris almost 125,000 second preference votes, meaning that Boris' first round lead was closed only marginally.
Before Janet opines further on the subject, I suggest that she might want to ask Mike Smithson of Political Betting for a briefing on how LibDem second preferences often divide quite equally between the major parties - though no doubt with some bias to the Tories in circumstances like those of last week's elections, in contrast to a strong bias to Labour in 1997 when the country was united in wanting to get the Conservatives out.