David Bookbinder , Dec 03, 2007; 06:33 p.m.
My guess would be that your main subjects were not filling enough of the focus sensor for the D80 to determine that this was what you wanted to focus on, perhaps because they were eight too small or too lacking in contrast, given the amount of ambient light. Can you post some?
Back focus would be a problem that would show up in all of your shots. This seems like an autofocusing / shot subject matter error.