Now, Boris's views are Boris's views – so we shouldn't read too much into the story. But it will still reinforce the idea that more and more Tories are coming around to AV. And it could fuel fears that the No campaign lacks a dash of oomph.
UPDATE: Boris's office gets in touch to point out that:
"Boris remains a keen supporter of FPTP and will not be voting for AV. It was never likely that he'd lead the campaign against it because he has a city to run.
Boris did admit to the Telegraph's Ben Brogan in April: 'The advantage of first past the post is that it delivers a decisive result. But that very virtue may be disproved. If it turns out that we wanted to kick them out and we didn’t, that is a big argument against FPTP.'
He was disappointed not to see a clear Conservative victory, but the system was decisive in 'kicking out' Labour, so he has not 'lost faith' in it."