Mountain Lion's version of Preview has a neat little ability that's tucked away under the Edit menu, labeled “Insert.”
As the name implies, this menu item will let you insert pages into a PDF that you're working on. You have three choices—Page from Scanner, Page from File, or Blank Page. Groovy!
First of all, I always find this much easier to work with if I turn Thumbnails on beforehand, especially if my starting PDF has multiple pages. To do that, pick View> Thumbnails, press Option-Command-2, or click on the View icon in the toolbar and select “Thumbnails.”
Yes, yes, I know. I always give you fifty million ways to do the same thing. So sue me.
After you've got your Thumbnails showing in the sidebar, click on a thumbnail to select it. Preview will put whatever you insert after your selected page.
Then just click Edit> Insert and choose whether you'd like to drop in a blank page, a page from your scanner, or a page from a file, and Preview'll do it.
Note, too, that if you're inserting something that has multiple pages, it'll just plop all of them in at once. If that isn't what you wanted, select the thumbnails of the pages you want to remove and tap the Delete key.
Easy, right? I would say “easy as pie,” but I've been wanting pie all morning, and none has magically appeared before my eyes. So pie ain't THAT easy.