Cairo's PDF/PS backends will be most appropriate for printing, but I don't think Cairo-generated Postscript or PDF would always be desirable for our "export" functionality.
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However, many designers also rely on EPS/PDF output as a "poor man's" interchange format as well. We at least want to provide an option for Postscript/PDF output that preserves as much of the structure of the document as possible, to enable meaningful import in other tools.
It'd be nice to separate the two roles somehow, since in the more common cases where editing isn't needed (e.g. for just sticking in a page layout), the Cairo exported EPS/PDF is probably going to be preferable.
Just for reference, that's actually how Illustrator handles PDF exports (and probably for the same reason). At least in AI 10, when you save as PDF, in the 'save options' dialog it has a checkbox that is to "keep Illustrator editing capabilities". So, a checkbox in a 'save options' or export dialog would seem to make sense. On a side note, it also has a radio button for which version of Reader you want it to be compatible with, that maybe we could have for which version of PDF you want.
-Josh