I have no idea about that, but probably not. Students that take the DAT during the summer have more time to study, so they probably do better. At least that is my hope.. I'm taking mine right at the end of the summer!
If you want comparison btw summer DAT and fall DAT, my exams were in Oct 2007 (1st) and in Jun 2008 (retake), and retake was much harder than the first exam. But, I think that I was just out of luck and got one of the harder version.
The degree of difficulty is more a function of how well you know the material on the test and less a function of the time of the year the exam is given.