Our US sibling site Gawker has a rather disturbing roundup of how Gmail mines through your contacts data in order to encourage you to integrate other social networking or email accounts into your Google account. If you'd rather not have that happen, here's how to fix it.
To switch off that option, go to the Settings link (in the top right corner when you're logged into Gmail), choose the Accounts and Import tab, and click on the link that says 'Other Google Account settings' in the first section:
This will open your main Google Account page. Under the Personal Settings section, click on 'View and manage your accounts from other services'.
(You should be able to get straight to this setting via this link, but direct Google account links can sometimes behave oddly, so I've detailed the full process as well.)
At the very bottom of the screen, there's a link that says 'Use my Google contact information to suggest accounts from other sites.' (as pictured at the top of the post). Make sure you deselect the check box next to this. When you do, Google automatically saves that preference -- there's no confirmation or save button to click.