The economist Paul Samuelson likes to joke that Wall Street indices predicted nine out of the last five recessions. So too in the new economy, where the fruits of success are often hailed as harbingers of doom.

Squeals of irrational exuberance could be heard only moments after the dotcom bust hit bottom. Google's initial public offering in August 2004, the first truly bullish news in years, splashed "Bubble 2.0" headlines across the media. Dire warnings again bubbled up in September 2005, with Ebay's $2.6bn (£1.4bn) purchase of Skype, the internet telephony service. Now, seeing Silicon Valley types "party like it's 1999" on the strength of social networking start-ups, even respectable business magazines feature stories grimly warning investors not to believe the hype.