To anyone puzzled by Hillary Clinton's tardiness in conceding defeat, distinguished US journalist James Fallows provided this explanation: 'The Clinton team doesn't worry about hurting Barack Obama's prospects of winning in the fall,' he wrote in the online edition of the Atlantic, 'because they assess those prospects at zero. Always have. Obama might not win if he leads a bitterly divided party, but (in this view) he was never going to win. Not a chance. He would be smashed like an armadillo in the road by the Republican campaign machine, and would be just about as ready as the armadillo for what was coming.'

Right on cue, a reporter from the respected McClatchy newspaper group travelling on the Obama campaign plane asked him about rumours on the internet of a video of his wife using a derogatory term for white people. 'We've seen this before,' Obama fumed. 'There is dirt and lies circulated in emails and they pump them out long enough until you, a mainstream reporter, ask me about it. That gives legs to the story. If somebody has evidence that myself or Michelle or anybody has said something inappropriate, let them do it.'

Welcome to the media ecosystem, in which 'mainstream' media have discovered that their new symbiotic relationship with the blogosphere sucks them into a downward spiral and an underworld of innuendo, lies and propaganda.

'Frankly, my hope is people don't play this game,' Obama told the McClatchy reporter. 'It is a destructive aspect of our politics. Simply because something appears in an email, that should lend it no more credence than if you heard it on the corner. Presumably the job of the press is to not to go around and spread scurrilous rumours like this until there is actually anything, an iota, of substance or evidence that would substantiate it.'

He's right, but the thicket of ethical questions raised by the new ecosystem engulfs us all. For example, should a responsible newspaper - or blog - provide a link to a dubious online source? The case for not doing so is that providing a link implicitly raises the status of the offending site. But in a world where Google will find virtually anything in an instant, such high-minded abstinence becomes an empty gesture.

As it happens, there is a way out of the morass, but it requires the application of old-fashioned journalistic skills and values. Or, more prosaically, sceptical, investigative reporting. The fact that something is circulating on the net is not, in itself, news - any more than is the fact that microbes circulate in drinking water. You can find anything you want on the net, and I mean anything. So what?

The rot that so offends Obama set in when 'mainstream' reporters began to relay what they found on the net in their own publications. And that happened a long time ago with the Drudge Report and the vicious right-wing campaign to bring down Bill Clinton.

A good example of how to deal with internet rumours was provided last week by David Weigel of Reason magazine. He went to work on Larry Johnson, the blogger who first started pushing the rumour about Michelle Obama. 'I know for a fact,' Johnson blogged on 26 May, 'that Barack and Michelle Obama would like the tape of her blasting "whitey" during a rant at Jeremiah Wright's church to never see the light of day.' How does he know? He 'learnt from someone in touch with a senior Republican over this holiday weekend that a major McCain backer has a copy of the tape'. Wow! Someone 'in touch with a senior Republican'.

Weigel tracks what happens next. On the night of 31 May, Johnson promises an update by 9am on 2 June. 'Now I know why people who have seen the videotape say it is stunning,' he burbles. On 1 June, Roger Stone appears on Fox News and says he's heard rumours about a Michelle Obama tape. At 8am on 2 June, Johnson reveals what he learnt from 'five separate sources who have spoken directly with people who have seen the tape. It features Michelle Obama and Louis Farrakhan. They are on a panel at Jeremiah Wright's church when Michelle makes her intemperate remarks. Whoops!!'

So the story whirls around the echo-chamber of the paranoid, right-wing blogosphere, with the odd whisk from Fox News reporters, until it reaches hysteria. But it all dissolves under Weigel's sceptical scrutiny, which is worth reading in full (see tinyurl.com/66k6ty). And no tape surfaces.

The moral? If confronted with online rumours, investigate first, report later.