I don't know beans about a DOF Preview on a Digital, but I know how they work (at least for me) on a regular film SLR.
Play with your apature then you may notice a difference. Maybe try the subject with a 2.8. then mayabe an 11, then maybe a 22 or something like that and you should notice a difference.
It will allow you to see what is and what is not in focus. For exmaple if you want what is right in front to be in focus you can blur the background and the DOF Preview will show you how it looks. Or if say you want to make the bars of a zoo cage melt away you are essentially blurring the cage out of the shot but the image behnd it is in focus.
Oh yes, I am very sad my current camera does not have this feature, I really love that feature. When I was in the camera store asking about the model I ultimately bought I asked about it and the guy looked at me like what in the world would you want that for? He was also a little critical of my wanting apature control (this was the camera store guy). Go figure.
Thanks for taking the time to read my post!----BlueDolphin