InDesign CS3 offers a new way to move document pages or master pages from one file to another, the Move Pages command in the Pages panel menu. It works like this:
1. Open two documents.
2. Select one or more document pages in one of the files, and choose Move Pages from the Pages panel menu.
3. In the Move Pages dialog, you will see a drop down list next to "Move to:". Choose the second file that you have open from this list.
4. Choose where in that document you wish to move the pages to using the "Destination:" drop down list.
5. Click the OK button.
The procedure above lets you push pages from the active file to any other open file. If you select one or more Master Pages in step 2, the Move Pages option in the Pages panel menu changes to Move Masters, letting you push the selected masters to another open file.
You can also pull master pages from another InDesign file (it doesn't need to be open) into the file you have open. To do this, choose Load Masters Pages from the Pages panel menu.
Unfortunately, you cannot pull document pages from one file into another. Document pages can only be pushed.