I just got the notification to update my Kindle Previewer 2.40 to 2.51. Oddly enough it said I first had to update to 2.41 because of a bug in 2.40 that prevented the update to 2.51. Amazon must have some of the most incompetent programmers in the industry!
After two updates I'm now running 2.51. It state that they added a "preview mode to emulate displays as on the Kindle Touch device" and fixed multiple bugs. Let's hope they did fix the bugs and didn't introduce a multitude of new ones like they normally do.