If dayhiking, I bring three liters and hope it's enough. If it's not, I risk a few mouthfuls from a source that meets certain criteria (high up, running swiftly, no dead moose) -- really, it's the gross-out factor.
Somebody on these pages said he drinks from up high, never treats, and hasn't had a problem. I got giardia at Katahdin and therefore don't entirely trust this theory, but if you run out of water, the risk of a few cupfuls is better than falling from poor coordination caused by dehydration.
If backpacking, I include the MSR miniworks. We used it to produce (what apparently turned out to be) potable water from one of the little streamlets crossing the Garfield Ridge (at which point we were VERY dry). The water was slow & muddy, but we got what we needed and didn't get sick. If pushed by conditions like these, I'll obviously risk it a little. But standing water like Carrigain's begs the question of what's falling in it, and filtering wouldn't address the gross-out factor for me, so I wouldn't go there.
It's this distrust of sources that makes me -- in a very suburban, yuppie way -- bring gallons of Poland Spring. It tastes good, it's safe, and it gets lighter by the hour. The only downside is that I go through a lot of it, and three liters is heavy in the morning.
