I've had some success with wort chilling in the cold and snow.
I've brewed outside in sub-zero weather and then put the brewpot in the snow. This, however, took a bit of work.
I have to pack the snow against the brewpot really, really well, and repack it every few minutes. And I stir often. It will work, if you keep at the snow and stirring. I can get my five-gallon batch down to pitching temp in 25-35 minutes usually. Be careful: the brewpot will sink through the snow and rest on the frozen ground (not a bad thing). But watch out that the the surrounding snow doesn't flip the pot's lid.
The lesson I learned: the cold air alone won't do it.
I did three batches using the snowbank wort chiller (since I didn't want to mess with hoses in these temps). Well, I gave in and got an immersion chiller. I'd rather haul in a pot of hot sub-boiling wort and use the immersion chiller at the kitchen sink. Still, though, there is something about putting on the Carhartt "wind bag" and brewing under the stars.
The salt water idea is excellent, though. I wish I'd tried that.
Good luck.