Firstly, set up a site-wide 301 redirect from non-www to www. There's plenty of example code in WebmasterWorld Apache forum here.
The danger comes in allowing both www and non-www URLs to both return '200 OK', rather than one of them correctly sending a 301 redirect OR once the redirect is in place, having any internal navigation links that when clicked send you through a redirect instead of directly to the correct content page.
In Google WebmasterTools, make sure your register and verify both www.example.com and example.com and that you look at the 'internal links' and 'links to your site' reports for both. It can be very revealing if you have a look a few weeks after making changes.