Does anyone know if there are standard conditions that are used to determine a sleeping bag's temperature rating? If so, are these industry standards, or do they vary by manufacturer?

What I'm getting at are things like: a) What is the tester wearing in the bag - a parka or um, au naturel - or somewhere in between? b) Is the bag outside or in a 4-season tent with the fly on top? c) How do you account for whether the average person would feel cold or uncomfortable at a certain point, as opposed to the tester; i.e., at what temperature is the comfort limit reached?

No, there are no standards for sleeping bag temp ratings. Some brands tend to be conservative, others rather optimistic in their ratings, and some vary from bag to bag. People vary too much to make ratings globally applicable anyways.

I generally divide bags into summer, 3-season, and winter, and the bags should be pretty clear on which they fall into. Everything else is magic, not science.

-dave-

I'm starting to see that. Been lurking over at cascadeclimbers.com lately and came across the following thread:
http://www.cascadeclimbers.com/threadz/showflat.php?Cat=&Board=UBB9&Number=560529&Searchpage=2&Main=560529&Words=&topic=1&Search=true#Post560529

Hoping my REI Sub-kilo 20 degree down bag will be ok on Rainier in July. Gauthier's book suggests a 20 degree synthetic for summer. I suppose an extra layer of clothing could always be put on, especially if we spend a night in the summit crater.