(From pdftk help (v1.41)):

background: Applies  a  PDF watermark to the background of a single input
		 PDF.  Pass the background  PDF's  filename  after  background
		 like so:

		 pdftk in.pdf background back.pdf output out.pdf

		 Pdftk	uses  only  the first page from the background PDF and
		 applies it to every page of the  input  PDF.	This  page  is
		 scaled  and rotated as needed to fit the input page.  You can
		 use - to pass a background PDF into pdftk via stdin.

		 If the input PDF does not have a transparent background (such
		 as  a	PDF  created from page scans) then the resulting back-
		 ground won't be visible -- use the stamp feature instead.