Just for clarity:
What we're talking about being missing from Photoshop CS6 is the preview near the bottom of the dialog that was there in Photoshop CS5 and earlier:

This is what you see now with Photoshop CS6:

They're both Operating System dialogs. It's just that Adobe has augmented the dialog to add additional preview functionality in the Photoshop CS5 and earlier dialogs and that has been removed for some unknown reason. Note that they have NOT removed all augmentation - that little [ ] Image Sequence box is not provided by Windows.
-Noel
[MODERATOR] Marking this post as correct as it perfectly illustrates the issue. This topic is further discussed here: Photoshop: No image preview on Open dialog box in CS6
Message was edited by: Brett N