The 2010 models are out, and Marmot really brought their A-game! It would appear that when converting to ºF, Marmot still uses * 9/5 part, but their researchers have come up with a way to eliminate the +32. For example, the new ratings for the Helium:
-4.3F / -2.4C
* Lower Limit
-15.7F / -8.7C
-48.6F / -27C
Forget Western Mountaineering and Feathered Friends - I'm getting a Helium! Under 2lb for a -15 ºF degree bag is really earth-shaking.
Unfortunately, if you take it to Canada or Europe you will find the bag not nearly as warm. Be careful if you're exploring the Rockies in northern Montana - cross the border by accident and you could be in for a very cold night!