So what threats are there? Google Personalised Search for logged in Google users is one obvious source of differences, as are regional differences from the different national search engines (e.g. google.ca versus google.co.uk).

With more and more browsers become location aware, I wonder whether we will increasingly see regional, or even hyperlocal, differences in standard web search based on browser location (something that presumably already exists in the local search engines).

Social signals (links from your friends or amplified by them) and real time signals also act as potential sources of difference for personalised ranking factors.

And for users engaged in a search session, the ranking of results you see in the third search in a session may even be influenced by the terms (and results you clicked on?!) in the first or second queries of that session.

Anyway, it seems that as of the weekend, there is another threat – perhaps a final threat – to that notion: Personalized Search for everyone:

Previously, we only offered Personalized Search for signed-in users, and only when they had Web History enabled on their Google Accounts. What we’re doing today is expanding Personalized Search so that we can provide it to signed-out users as well. This addition enables us to customize search results for you based upon 180 days of search activity linked to an anonymous cookie in your browser. It’s completely separate from your Google Account and Web History (which are only available to signed-in users). You’ll know when we customize results because a “View customizations” link will appear on the top right of the search results page. Clicking the link will let you see how we’ve customized your results and also let you turn off this type of customization.

Chris Lott also made a very perceptive comment:

PS It also looks like Google are looking for even more traffic data to help feed their stats collection’n'analysis engines: Introducing Google Public DNS

PPS it seems that Google just announced real time search results integration into the Google homepage. It’s still rolling out, but here’s a preview of what the integration looks like:

Read more at Relevance meets the real-time web. Exciting times…

PPPS Seems like there’s no global, or necessarily even national, ground truth in Google Suggest results either: Google localised Suggest…