Brian sent us (thanks!) what is probably the very first Keynote '08 hack. He found that there's a new stroke option in the program called Picture Frames, which are various types of framing graphics to put around a slide. The standard choice screen lets you choose from 12, but Brian opened up the Keynote files, and found that there were actually 72-- apparently Apple included Frames for its own Themes, but didn't allow the user to use them separately.
Fortunately, all it takes to show the extra frames is their names in an xml (actually a .plist) file inside the Keynote .app folder, so Brian cracked it open, and added all the names. Turns out, also, that no matter how much you stretch it, the Picture Frames choose can only show 50 at a time, so Brian even created a customized .plist file that shows his 50 favorites-- all you have to do is download it from him, put it inside your Keynote .app file, restart Keynote, and you've got access to almost all the Picture Frames that Apple does.
It's pretty clear why Apple didn't want people playing around with custom frames too much (opening a Keynote file with a custom frame requires that you have it installed, apparently, so sending a Keynote file to your friend without the custom frame could lead to trouble), but who knows why they didn't at least give you the option of using all the frames everyone has. Thanks to Brian's superfast hack, now you can.
Update: Just to make it clear, it's fine to use these hidden frames, since everyone has them installed. The problem with sharing frames comes when you send someone a slide with a frame you created-- they don't have that frame installed, and so they won't see it.