I was asking myself this question after reading Colin Hazlehurst's
post on the topic at Inside epub. Hazlehurst suggests using an additional rootfile linking to a PDF to provide a version of the pub formatted for printing. I tested a file with two rootfiles, one pointing to a PDF, in desktop versions of number of readers (ADE, B&N, Stanza, etc.) but none revealed the presence of the PDF file.
I've been toying with the notion of including a link or script in one of the content files that would call the PDF but haven't yet figured out if that is possible. Any suggestions welcome.