Just found an interesting article on this topic:
https://www.justlanded.com/english/Canada/Articles/Visas-Permits/Permanent-resident-cards
If you're from a country that requires a visa just to visit Canada, then as others have stated, you shouldn't leave until you have your PR card. If you absolutely have to, you can leave and then apply for a travel document from outside Canada, though it will take probably several weeks to be processed.
However, if you're from a visa-exempt country, it seems like you could risk it. Legally speaking, "[CBSA officials] will probably chide the traveller for travelling without their PR card, but no laws will have been broken by the traveller". Of course, it'd be up to the individual to decide if they want to take the risk of facing hassles upon re-entry.
FWIW, I have an Australian passport (ie am visa exempt), live & work in Canada, have been waiting for my PR status since early March 2014, and have flights to the US booked for end of June. I should have PR status by then (I'm "in process" at the moment), but if I don't have my PR card, I will risk leaving Canada. I'll bring printouts of correspondence from CIC to show upon my return, my PR application number etc, and the officer will just have to look me up in the system. I guess worst-case scenario they only let me back into Canada as a "visitor" to continue waiting for my PR card to show up in the post after another week or two.
