lammert - 7:09 am on Jul 16, 2010 (gmt 0)
Google knows that sitename.com and www.sitename.com exist. In the past the algorithm tried to determine on its own which was the preferred domain and that didn't always went right. Some sites were indexed with half of the content as www.sitename.com and the other half as sitename.com. Actually one of the measures they took to prevent that problem to happen was to ask webmasters to add both domains to GWT and use the preferred domain display setting. You should be safe having both domain versions in GWT, as long as you make sure you use the preferred display setting.