The 'denial of service' attack which brought Yahoo! to its knees last week prompted much editorial spluttering about the dangers of so-called hacking and e-terrorism. 'The fact that Yahoo! was taken down means,' said one alleged security expert, 'that nobody is really safe'. (Yahoo! is one of the biggest sites on the Web in terms of the number of virtual visitors it attracts every day.)

Denial-of-service attacks attempt to overwhelm a server by flooding it with timewasting requests. Up to now they have been easy to detect and relatively simple to counter. But what made last week's attacks problematic was that the deluge of requests came not from a single source, but from about five sites around the Net. It was reported that at one point, the Yahoo! servers were trying to deal with bogus traffic reaching a thousand megabytes a second.

The threat of such 'distributed coordinated attacks' has been well known and widely discussed for months, so it was only a matter of time before someone got their act together. The day after Yahoo! was hit, four other prominent sites were struck. The trouble is, the only way an e-commerce site can be completely safe is if it shuts down and disconnects from the Net, which rather undermines the case for getting into e-commerce in the first place.

Then again, if lossmakers such as Amazon went off-line for a while, think of all the money they would save! Maybe that will be the next twist in the dot.com spiral - companies shutting down their sites and seeing their share prices go even further through the roof.

What the humiliation of Yahoo! demonstrates (not to mention the rout of , whose share price has plummeted since its misguided attack on a group of Internet activists) is that the online community can be a fearsome opponent. The corporate suits and entrepreneurs who see cyberspace as the next Klondyke overlook this at their peril. The suits come from a culture where those with the most expensive lawyers and the deepest pockets tend to come out on top.

But this doesn't hold on the Net, where the collective IQ of the programming community far exceeds anything the corporate world can muster. Most so-called 'hackers' are not much enamoured of the gold-rush mentality which is currently running riot in Wall Street and the City. Au contraire: many of them rather like the idea of the Internet as a meritocratic commons - what writer Richard Barbrook memorably called 'a high-tech gift culture' - rather than a capitalist jungle.

So whenever the corporate world starts throwing its weight about on their patch, it can expect the same kind of trouble as brought Yahoo! down last Monday.

• A business asks me whether he should be worried about the Net. I reply that unless he is an undertaker the answer is yes.

As if to underscore the point, the Ford Motor Company has announced that anyone wishing to do business with it in future will have to do it via the Web. Daimler-Chrysler is following suit, and you don't have to be a rocket scientist to understand why. It costs these companies something like $65 to process a paper-based transaction, but they have to pay less than $5 for the online equivalent.

Now the message has crossed the Atlantic. Shell Services International, for example, is putting all its procurement online. And National Power, having discovered that it was costing £75 to purchase £50 of stationery, is now buying online at an average cost per transaction of less than £10.

The time is rapidly approaching when the most fervid Internet evangelist in a company will be the purchasing manager.

• john.naughton@observer.co.uk