Sitecore v6 introduced another preview tool accessible from [Presentation] > [Preview] as opposed to [Publish] > [Preview]. The new preview mode opens another tab in the content editor which shows the preview of the current item.
This is all fine and well except that it is permanently looking at the web database rather than master database. This can be observed by creating a new item, saving it, and then selecting preview. You get a 404 error, unless you publish it first.
Unfortunately this nice feature is 80% broken and can only be used for looking at published content.
Update: This was a registered bug in February 2009 with a workaround available from Sitecore support, but this still has not been released and I believe the bug still exists in Sitecore 6.1