Inside the 10.5 File » Open dialogue, as you probably know, you can use the Media category in the dialog's sidebar to browse iPhoto or Aperture photos. After you select an image, press the Space Bar to invoke a zoomed image view, showing the chosen image in a larger size. Once zoomed, you can continue to use the arrow keys to navigate your library while seeing the images in zoomed mode.

[robg adds: Given that this works with photos, I thought I'd try it with the Music and Movies sidebar entries, too -- and yes, it works for both of those media types (videos play within the dialog, and songs play, when you press the Space Bar) as well. So it seems it's basically like Quick Look in the Open dialog. I tried it with files outside the Media section, however, and it doesn't work there -- instead, it seems to toggle the Preview column between detailed and simple views.]