Postscript: 11/15/2014. Since writing this blog, I've found a section right at the end of Milne's "House on Pooh Corner" (1928) which is no doubt the origin of the quote above. Christopher Robin is leaving Pooh (presumably to grow up; not because he has a terminal illness), which they discuss, ending with:
"Pooh, promise you won't forget about me, ever. Not even when I'm a hundred."
Pooh thought for a little.
"How old shall I be then?"
"I promise," he said.
Note: the conspicuous absence of anything about dying, because that would be a very depressing way to end the last page of a children's storybook. Also note the absence of the phrase 'a hundred minus one day' because neither Pooh nor Christopher Robin say shit like that.