For those thinking about traveling while their PR application is in progress I wanted to share my experience. My partner and I (sponsor) recently traveled to Europe for a couple weeks to my partner's home country to visit family and friends. We are still waiting for AIP and my partner is currently on a visitor's visa.
On our return home, we went through customs in Montreal, there were no issues at all. The only thing the officer was concerned about is making sure my partner had legal status in Canada. In her case she's has on a visitor's visa and that the was only thing the officer asked to see. I'm not sure if he actually checked that we had a sponsorship application in progress; the officer let us through fair quickly.
For people thinking about traveling outside Canada during the sponsorship process I would make the following suggestions.
1) Limit your travel time to about 2-3 weeks (typical vacation length).
2) Travel with your sponsor.
3) Don't travel if you let your status run out.
Make sure your bring the proper documentation - current visas, AOR letter from CIC etc.
Thanks to the people who gave me their info when they traveled outside Canada.