When viewing photos in the grid layout of iPhoto '08 running on Leopard, simply press the spacebar to zoom the selected image to full size of image browsing part of the window. Press the Space Bar again to return to the grid view, or use the Left and Right Arrow keys to scroll through your images.
Of course, this seems intuitive if you use QuickLook in the Finder a lot or use the this hint for a similar effect in the Media Browser, but it's not documented anywhere so I hadn't tried it.
It's a huge help to use this function to quickly look more closely at a few images while still taking advantage of the normal grid view to quickly browser through the rest of the image library. No more switching to full-screen mode or zooming in and out just to examine a few images.
[robg adds: I don't think this is exclusive to iPhoto '08 or Leopard; I recall it working in older versions of iPhoto as well, but perhaps that's a false memory.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081029204300418