Preparing sibling for birth process?

We'd like to have the option of having the Bug (4.5 y/o) present at
the birth when the little dude is born. My mother, who is very
involved in her life, would be at the hospital as the designated Bug
manager so that they can go in and out of the room as seems
appropriate. But of course I wouldn't want to spring this on her
unawares, and am more mystified and apprehensive about how to open the
discussion with her about the (relatively) gory details of birth than
I am about actually having her there. I know a lot of people on these
lists have had older siblings present at birth and am curious about
any good resources you know of (books, videos, whatever) to educate
preschool age children about birth. Growing up as a doctor's daughter
she's generally knowledgeable about the body and enjoys looking at my
rather graphic medical school anatomy textbook. She particularly
likes the drawing of the abdominal wall splayed open to show the
stomach and intestines -- point being, she's not squeamish about this
sort of thing, so I could see a non-kid oriented photographic book
about pregnancy and birth also serving as a good tool to prepare her.

Kate, ignorant foot soldier of the medical cartel
and the Bug, 4 and a half
and something brewing, 4/08