(I posted this about 2-3 weeks ago, just as my campus server crashed. Usenet has been down (here) since then, so I don't know if this appeared or not, or if there were any responses.)
I'm aware of a common rule of thumb saying something like slopes less than 3 or 4 in 12 should not have shingles. I live in a development where all the homes are 25 years old. In the last 5 years several neighbors have replaced their roofs; even on slopes of 1 in 12 they have all used asphalt shingles, the same as the rest of their (steeper) roofs.
I plan to have my roof replaced this summer, and so far have been unable to learn of a code REQUIREMENT, in contrast to "general knowledge" about there being a minimum slope for asphalt shingles.
I'm located in Southern California, in an area that gets very little rain.
Any knowledge, advice, and/or insight appreciated.