Giardia tolerance of a sort can happen. People in parts of the world without a treated water supply (aka 70% of the planet) are gulping the little buggers down all the time. Like a previous poster said, it all depends on the concentration and nastiness of the bacteria and on the state of your immune system and the flora in your guts. Lots of those people drinking their neighbors' pottywater had acute giardia at some point(projectile vomiting/diarrhea), survived it, and now have settled down into chronic infections, which let them get on with their lives, give or take the occasional shart or liquid burp. What those people also have are infant and elder death rates that are through the roof.
My advice? Go ahead and take the occasional liberty if you're in an area that seems pretty safe and you just have to taste that Rocky Mountain melt, but know that eventually the shits will strike, and strike hard. Or not. Are you a lucky guy?