Great summation, Melanie.
I've been reading up some on WLS patients - people who have weight loss surgery (Jiffypop could probably pop in here with some better info). For the first year or so after surgery, their average daily caloric intake generally averages less than 1,000 calories - and they still lose weight (as long as they stick to their plan - it IS possible to out-eat the surgery, from what I've seen and read).
And of course for 12 weeks in 1990, I was on a 800 calorie a day liquid fast, and lost pretty rapidly - but I do remember the first week of the fast I lost something like 8 lbs on the scale (mostly water of course!) and the second week less than 2 lbs, even though I didn't cheat or anything.
Many WLS patients, right after surgery, plateau and don't lose weight for a few weeks.
I wouldn't call either instance a case of "starvation mode" - I'd call that a plateau or body adjustment.
I'm sure there are studies of concentration-camp survivors - many of whom lived on FAR less than 1000 calories a day (from what I've heard, probably less than 500 calories a day) while being forced to do hard labor AND having to survive extremes of hot and cold weather, with very little clothing to protect them. THAT is what I would call starvation mode.
Come to think of it, I've recently read about studies being done that show that reduction of calories (what we would probably consider a SEVERE reduction of calories) might actually increase one's lifespan! (I read about it in Time magazine a few months ago...will have to dig that one up).
IMO - the "Starvation mode" theory is JUST a theory.
However, there is a lot to be said for zig-zagging your caloric intake - instead of eating the same amount of calories day after day. Zig-zagging calories might help you break the plateau, if you're on one - there are a lot of plans that implement zig-zag, including
Burn the Fat Feed The Muscle.