Anyway, Mountain Hardwear's Phantom 32D ($240; www.mountainhardwear.com) is a superlight down bag that is rated to 32 degrees and weighs a paltry one pound, five ounces. Of course, that's exactly what you want for this sort of hike. It's a superb little bag.
So what's the concern about down? That it will get wet? Always possible, of course. But I assume you'll be carrying some sort of lightweight tent or other weather shelter. So that will keep most moisture off the bag. And you'll always want to toss it out in the sun when you have a chance. That's just good bag-care policythe bag will soak up a pint or two of body moisture each night, even without dew or rain. So you want to be sure that moisture evaporates.
As for the Phantom's shell, it's a tight-weave light nylon that's treated for water repellency. Not waterproof by any means. But light splashes and maybe a little rainfall through the tent door will run right off.
In short, the Phantom is the sort of bag I'd take on a hike like that. I don't think you'll have any problems with it at all!