I've implemented a small patch which allows to automatically set the overprint state for text in pdf output. This is useful when text and background colors are printed on different printing plates because it ensures that no white spaces are visible if the printing plates are not positioned perfectly.

See also: http://www.nabble.com/Overprint-Black-Text-td9852903.html

Feel free to review or integrate it into fop trunk if it's for any use to you.

Usage:
foUserAgent.getRendererOptions().put(PDFConfigurationConstants.KEY_ENABLE_AUTO_OVERPRINT, Boolean.TRUE);