Exporting to PDF (Print) results in a hung background task that cannot be cancelled. If you attempt to quit InDesign, you get the message, "InDesign cannot quit because one or more tasks are running in the background."
Install the Iatest InDesign updates by choosing Help > Updates, or by visiting the Adobe product updates page: http://www.adobe.com/downloads/updates
Note: For a startup script that provides a menu choice to control background export, see this link.
InDesign CS5 introduced PDF export as a background task via multi-threading capabilities. Some third-party and InDesign native operations become confused by this new method and result in a hang.