M_Bison - 12:18 pm on Jun 21, 2008 (gmt 0)

Now, this could be as a result of clickthrough data, however I don't think it's that simple. Google.com results do not list google.co.uk in the top 10 at all, and so where would the clicks come from?

From google.co.uk - if Google sees one site being clicked on more on google.co.uk, they will increase the rankings on google.co.uk and NOT google.com. In fact, the site may not even rank well in google.com.

I think a factor may be the 'regionality' of a search - i.e. how likely it is that the visitor is looking for a region-specific resource.

Definitely. You won't see any US based firms on google.co.uk for searches relating to things such as insurance or finance, and increasingly for searches relating to e-commerce (eg where the searcher is looking to buy). However, for searches like "second world war", you'll get a mix of results from UK, US and other countries.