With personalised search a new layer of complexity has been added to getting your site to rank well and also raises a whole host of data protection issues.
Google personalised search goes back to what I've been saying for the past three / four years - that Google is focusing on immediacy and may have taken their eye of relevancy. But now it's gone full circle and they're using the full range of information they have to try and get the relevancy issue sorted again.
For the past few years they've been trying to guess exactly what you're looking for in terms of are you in the buying or information retrieval frame of mind.
If you take a search for "marketing jobs warrington" you get the GBase stuff which tries to get you to those jobs as quickly as possible:

If you do a search for "Sony Vaio" you get Google products search results - because it thinks you're in a buying mood:

But to cover themselves, they've also added deeplinks into content on the Sony site - they've correctly matched the search term to the relevant authority site.
Now if you take a search term that they've twigged that most people are actually looking for a picture about - they give you some image search results at the top of the web search results:

Now, if you're looking at a purely informational search say for "diarrhea" (I hope you're not looking for images of it!) they've nicely categorised the possible nature of your search:

So we know that Google uses their vast array of search data and then categorises your search task (find information, find images, find products etc) and then applies that set of beliefs to your historical search pattern.
But where else can they get more information to refine YOUR serps?
If you use a personalised
Google Homepage or
Google Feed Reader will they look at what feeds you've included?
Would they go far as to read the nature of your Google Mails (as they do to serve relevant ads)? If you get loads of email announcements from Gadget shops will they think you want to see gadget shops ahead of information ones?
If you've got a loads of sporting events in your Google Calendar will they use that to refine your search results?
Some people use Google Notebooks, if you tag pages will they be moved up your own search results?
You could even go as far as what you've seen on Google Video & You Tube, which Newsgroups you're subscribed to etc.
What makes it more "interesting" is that they could read your Google Docs (I use them a lot for planning and client interaction) and even what's on your desktop (using Desktop search).
If you use
Google Page Creator, will your search results be refined by the pages you create?
How far will they go and how far do we want them to go? I have no problem with them using certain amounts of data to provide me with more relevant results. But each individual should be able to opt out of each source.
Some people don't even know their Google Mail is scanned for themes and some people trust Google not to miss-use the data. However, there are some that are concerned that any amount of individually identifiable data that is transmitted across borders is illegal which form the
8th Principle of the Data Protection Act:
Personal data shall not be transferred to a country or territory outside the European Economic Area unless that country or territory ensures an adequate level of protection for the rights and freedoms of data subjects in relation to the processing of personal data.
from my knowledge the USA isn't one of those nations that complies - is our personally identifiable information crossing borders without our knowledge?
It's the second principle that worries me, however:
Personal data shall be obtained only for one or more specified and lawful purposes, and shall not be further processed in any manner incompatible with that purpose or those purposes.
Are people totally aware that their search results are being used to refine searches or that other people logged in could in effect change their search results? Then you get into the areas of Google docs, desktop, notebooks information being used to sort results. There could be addresses, telephone numbers and gawd knows what in there.
I really hope that Google address these concerns that I expect many others have, because any "big brother" connection could seriously harm people's perception and use of Google.