To change this, go to Pages > Preferences. Under the “General” tab, you’ll see “For New Documents.” Toggle the “Use template” radio button (or, in Keynote, the “Use theme” one), and then select “Choose.” This will allow you to pick whichever template you use most often, including one of the blank ones. After making your selection and hitting yet another “Choose” button at the bottom, you can close the preferences window, and you’re done. You can then feel free to make Numbers and Keynote behave correctly, too.
One of the most interesting aspects of this is that you can create your own templates and instruct the iWork applications to always open with those, which is great to use with letterhead, Keynote presentations that always have the same design, and so on. After you’ve edited the document to look the way you’d like it to, choose File > Save as Template (in Keynote, the command is File > Save Theme). You’ll then be asked what you’d like to name your new creation. An important note here is that you’ll want to leave your theme or template where iWork tries to save it (within your home folder’s Library > Application Support > iWork > [application name] > [Themes (or) Templates] folder). If you don’t, you won’t see your document appear within the Template Chooser if you do decide to open it that way.
Pages’ terrible Template Chooser. But hey, at least my custom ones are available now!
After you’re done editing and saving out your masterful letterhead (or what have you), you can repeat the first steps I listed to set it as the default when that application is opened or when you hit Command-N. What could be easier? Well, not having to do it in the first place, I suppose, but we don’t live in that perfect world, you dreamer.
My attractive and VERY useful new Keynote theme.