Based on a previous blog post, I have discovered the location of the hidden Picture Frames used in Pages '09 (and the other iWork '09 apps), and the plist file that determines which ones are accessible. You can use Picture Frames by selecting an object in Pages, opening the Inspector, clicking on the Graphic button, and choosing Picture Frame in the Stroke drop-down menu. The Picture Frames that are available by default leave out a bunch of Picture Frames that are installed as part of Apple's templates.
You can view the installed Picture Frames on your machine here: /Library » Application Support » iWork '09 » Frameworks » SFRendering.framework » Versions » A » Resources » Frames. (This link may show you the folder, depending on which browser you're using.) You can enable any of the picture frames shown in the above folder by editing a plist file within the Pages application. Open the iWork '09 folder, then Control-click on Pages and choose Show Package Contents from the pop-up menu. In the new window that opens, navigate to Contents » Resources » SLGraphicInspectorFrames.plist, and open it in your favorite plist editor. Again, this link may work to open the file on your machine.
Note that the name of each frame to be added to this plist file can be found in the index.plist file of the individual frame.
For example, for the Roadtrip Receipt frame, navigate from the Frames directory down into Roadtrip Receipt.sfrframe » Contents » Resources, and open index.plist. There you'll find the key named name, which will show RoadTrip_receipt as the value. That value is what you should add to the SLGraphicInspectorFrames.plist file to enable the Picture Frame in Pages '09. Note: This same process would likely work for Keynote and Numbers as well, but I haven't tried it.
Mac OS X Hints