The latest version of Google Docs lets you add headers and footers in your documents. Just go to the Insert tab and click on the appropriate link. Google Docs still doesn't provide support for pagination, so you can't add any information about the number of pages in your header or footer. In fact, there doesn't seem to be a way to enter dynamic content.
In November last year, Zoho Writer added a special view for pagination and more advanced headers & footers. While the page view is still read-only, Zoho gives some idea of your document's layout. In Google Docs, the only way to see an estimation for the number of pages is to select File / Word count.
If you try to print a Google Document, you'll have another surprise: Google will export your document to PDF without mentioning that or adding a link to a PDF reader. In the print settings you finally have the option to add page numbers, but make sure there's no conflict with your document's header or footer.