Google's recent decision to start selling cost-per-thousand advertising came as a bit of a surprise. Widely known as CPM advertising, this approach is about "eyeballs" rather than click-through rates. Advertisers can opt to pay based on the number of times their advertisement is shown as part of a page, rather than the number of times a person actually clicks on an advert.

The new CPM ads will not be carried in the regional versions of Google's search services, but in the network of affiliate sites worldwide that carry Google adverts. But Google's move - and its gigantic presence - will bring a new focus to the hazy world of online advertising.

As you may know, many sites that carry advertising are audited - so that advertisers can have some certainty about the audience for their ads. It's easy to claim a million visitors a minute, all of whom are interested in buying high-value goods, but proving it requires a bit more than showing off the smoking server stack.

In the UK the best known auditing body is probably ABCE - the Audit Bureau of Circulations Electronic - but business-to-business auditor BPA also carries out online audits. Both have their roots in the very different world of verifying print circulations.

The problem for auditors, advertisers and site owners is that while it's easy to verify visitor numbers, there is no good way of assessing who users are, how often they visit, and whether they arrive by mistake. Few site visitors want to log on unless cash changes hands; IP addresses are rendered untrustworthy by address translation and dynamic allocation; and privacy issues aside, many browsers now quietly ditch cookies at the end of every session. These factors make it hard to demonstrate what kind of audience a site attracts.

Google, however, is in a unique position. Such is its dominance of web searching that at many sites most of the people who don't arrive by clicking a bookmark will arrive via its engines. Google accounts for 30 times as many visitors to IT Week's site as Yahoo, and 100 times as many as MSN, for example.

And if Google sends people, it also knows what they are after. From an advertiser's perspective, Google is actually assembling relevant audiences on the fly, and then putting them in front of pages that could carry ads. This is a very powerful proposition.

As Google grows, it could well make traditional auditors look like amateurs. The agencies that place ads online are also likely to look a little redundant, since they are in the business of optimising audiences by choosing among sites that Google sends visitors towards. The likes of DoubleClick would then be next for the chop.

In short, Google could turn its domination of searching into hegemony over online advertising.

Of course that's just my theory. But if I made my living as a middleman in the online advertising chain, between content provider and advertiser, I'd be worried about the future.