Conduit SL Bivy
But camping out in a "bare" sleeping bag isn't the best thingas you note, the bag gets pretty wet with dew and frost, and drying it out is apt to be tricky (I take it you're really camping, and not just sleeping in the backyard because you've insulted the missus; otherwise you of course could just run the bag through the clothes dryer for a few minutes). Moreover, only a very few sleeping bags are truly waterproofall the required stitching renders them full of tiny holes through which water readily seeps.
What you need is a simple sleeping-bag cover, which will both keep the bag dry and add a few degrees of warmth for those especially frosty nights. One good choice is Mountain Hardwear's Conduit SL Bivy ($110; www.mountainhardwear.com), made with that company's proprietary waterproof-breathable material. It's totally waterproof and will keep you and your bag nice and dry. And, it weighs just over a pound so won't add a lot of weight to your sleep setup. A similar item is Outdoor Research's Basic Bivy ($179; www.orgear.com), which uses Gore-Tex (hence the higher price), but like the Conduit SL Bivy is simply a seam-taped bag into which you slide the sleeping bag, then climb in. It's not bad for warm-weather use as a standalone, either, as it has a hood with bug netting.
If weight really is an issue, the lightest bag cover that I'm aware of is MontBell's U.L. Sleeping Bag Cover ($160; www.montbell.com). It's made of two-ply Gore-Tex and weighs just a touch over seven ouncesyou won't even notice it in your pack!
Finally, you might also consider a hammock, which will keep you off the snow-covered ground (OK, I reassert my claimyou're mad). Hennessy Hammock makes a selection of hammocks that are well-regarded by the ultralight crowd. Try the Hennessy Ultralight Backpacker ($170; www.hennessyhammock.com), which will keep you snug and off the winter permafrost.
For more expert reviews of the best sleeping bags, check out Outside Online's Sleeping-Bag Buying Guide.