I have the same problem with my black Macbook. I was startled today when it just suddenly shut down with no warning (no low batt warning either). I had not noticed that my Magsafe had popped out so I was on batt without realizing it. At the time, I had only Preview, TextEdit and Firefox open. I had been using wireless to net earlier but the Airport card was turned off. I was in the TextEditor when it shut down.
I plugged it in, waited a minute, pressed power key, machine booted rather than waked. It booted as normally as if I had shut it down, nothing was restored as is the case when the machine sleeps. Firefox, when I launched it, did act the way it does when you shut down without closing the browser, i.e. asked if I wanted my session restored etc.
June 15th is when I took this machine to 10.5.7 and it's entirely possible I have not used it down to a low batt situation since then, until today. The machine only has 62 cycles on the battery. The battery health is characterized as good and the full charge capacity is listed as 4902.
I do know that my battery qualified for that firmware update and it was applied on June 15th as well. I am wondering if that thing somehow altered something that is relied upon for the low battery warning.
Well anyway now I'm turning autosave back on in TextEdit on this machine, since APPARENTLY I have found a use for it...