8:48 p.m. on March 21, 2008 (EDT)
Your grace, who, pray tell, is B ... oh, never mind, I don't care anyway.
Like kutenay, I use what works. WISam is worried about the bag getting wet, and he wants it compressible and light. At the temperatures you are talking about, Sam, you don't have to worry about down getting wet. As long as you are below about +10F, a quality bag with a microfiber outer will be water-repellent enough to allow you to brush off any spills of your tent-mate's soup or coffee without it getting to the down. And a microfiber outer will breathe well enough so that the approx 1 liter you sweat off during the night won't condense in the down. Waterproof shells, such as Goretex or Gore's Driloft, don't breathe well enough, as Will Steiger and his crew found out on their North Pole trek - their bags weighed 75 pounds by the time they got to the Pole and were not stuffable, thanks to the condensation and freezing from body perspiration condensing on the inside of the waterproof/"breathable" shell.
Synthetics in general, will not meet your light/small criteria, and they don't have much lifetime compared to down. The one exception at present is Primaloft, which is about equivalent to 500 fill down in its latest form. Still, it seems to be limited to a 15 year lifetime, compared to 30 or 40 years of hard use for down (I still use my 1960 Karakoram from time to time).
The companies that kutenay mentions are NOT big companies. I would add to the 4 he listed, Feathered Friends, which is based in Seattle (and makes its bags in Seattle). They are one of the few companies around that will customize their bags (Western, located about 10 miles from me here in the SFBay Area, stopped doing customization several years ago). Barb and I have -40 bags from them. I have had my current one for about 8 years, during which I have used it for at least one 20+ day expedition and several backcountry ski tours each year for the last 8 years (my previous one had a slight problem with the Gore Driloft in the hood area delaminating, so FF replaced the bag under their lifetime warranty, using another waterproof product in the hood for the breathing tunnel - keeps your exhaled, very moist breath out of the down, something most companies don't even think of).
Kutenay's suggestion of a VBL is an excellent one. The one I use is the Integral Design 4 ounce VBL/emergency bivy. The VBL will keep the moisture from your perspiration from condensing in the insulating down. I will note that some people do not like VBLs, feeling they are "clammy". So you need to try it to see if you can get used to it, or if your body adjusts its perspiration rate to the humidity in the bag.
The one place you might have problems with wet bags if you do not learn the simple precautions is if you are sleeping in a snow cave, quinzhee, or igloo. These will get up to within a few degrees above or below the freezing point inside, and you can have dripping from the roof of the shelter if you have not properly shaped it. In this case, a bivy sack will suffice to keep the down bag protected.
I have a -10F synthetic bag that is not Primaloft, and it is way too heavy for any distance into the backcountry - 5.5 pounds, and requires an XL stuff sack. My -40 FF bag (and Barb's, too) is about 3.7 pounds and stuffs easily into a medium ID Silcoat stuff bag, which I then compress.
Yes, these bags are fairly expensive. But they last for decades of hard use, and they WILL keep you warm.