Brian, who also told us about that Keynote hack when '08 was first dropped, sends a note about Galleria, a theme he's selling for Keynote that's designed specifically to show off photos. The theme takes advantage of the new "media placeholder" function in Keynote '08 that lets you place photos exactly how you want them, and then replace them with just a click. And any photos you put in can then be resized or cropped right there on the slide.
Galleria also has lots of samples to use, layout choices, and extras like shapes and buttons and picture frames. It might be a little more than you need unless you're putting together a Keynote project that shows off lots of photos, or if you regularly show off photos in Keynote presentations (it's $19.95 for the whole package). But if it fits your bill, Brian definitely knows what he's doing with Keynote-- using his theme could save a lot of time in getting things set up yourself.