I have been using a technology similar to this from a British company called Blizzard Protection Systems: http://www.blizzardprotectionsystems.com/acatalog/index.html
The technology is called "Thermocell" and the Blizzard Bag that uses it costed me $30, weighs about 11oz, is waterproof, and according to them has a thermal efficiency of over twice that of down: http://www.blizzardprotectionsystems.com/reflexcell/reflexcell_7.html
I can attest that this is mostly true (albeit to a low fill power of down). I've taken this bag alone with no clothing other than a t-shirt and boxers out on many summer nights, and have even been toasty warm and had to vent it. Of course, these were 50-60 degree nights.
Tomorrow (Saturday) night I will be testing it at 10350', and I will bring my Arc Alpinist as a just-in-case, which I plan to use as a liner for a very warm night if needed.
The bag has 3 layers, and the first 2 are supposed to be breathable. The directions indicate to air it out for 48 hours after use. I found that the 2 layers do seem to absorb the greater ammount of moisture, but some did still condense in the foot box area in my tests.
Overall I think this bag is a killer deal. I've been able to ditch the bivy because of its shell and in the winter plan to use it as an 11oz super-booster for added warmth. This would only add 4.5oz extra weight to the bivy I'd carry if not the blizzard bag.
Note that they also make a jacket and a few other garments that might be worth trying.