I've been into Canada about 20 times (I think) over the last 4-5 years. I've been stamped twice: Once during the ritual Customs grilling when I got pulled off the train (it was autopilot for them to stamp in that case, I think) and once when I was doing an in-and-out on the Portland-Yarmouth ferry (I was on Canadian soil for about 90 minutes between docking and reboarding).

I think part of the reason the US and Canada have a no-stamp situation is that you've got plenty of folks for whom those stamps would fill out a passport book every year.