Ya man the title says it all. Of course a laptop will shut down when it's battery is empty. Freakin' Duh.
It will tell you that you are running on reserve power, and that means that perhaps you should take the hint and plug it in or at least save your data.
When the battery really is that low on charge, it will not have enough power to retain the data in memory, so me thinks that Apple coded it in so that it just shuts down normally when you get that low, so as not to have it have a hard shutdown.
I would think that the difference between your iBook and your MBP is that the MBP will let you run more or less until it cannot run any more, and hopes that you are of enough intelligence to take the hint when it keeps reminding you that you have no charge left, and will push itself to where it has a true 1-0.5% charge left, than kills itself. I'm guessing that the iBook would sleep itself when it truly had something to the extent of 1-2% charge left.
Moral of the story: things change from build to build, and there is a reason that that prompt tells you to save data and plug it in ASAP
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Not to mention the fact that completely killing a battery cannot be good for it's health...
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