Powell - That sounds like my mother!! You shouldn't have to justify anything to anyone about when you are going to post stuff on facebook or why you don't want other people doing it yet! I just got into a discussion with my mom yesterday about a cute little announcement picture we are going to do on facebook.. and that she wanted me to warn her before we do it, so she can tell people... I am like no... I don't want you telling anyone (except for a few close family friends that I have approved), until I have made the announcement.
I say let her throw the pity party, because ultimately you have the upper hand... you have the grand baby.. she will come around, and if not, just ignore her.
Speaking of facebook settings, you can also turn off the ability for other people to post to your wall as well. Might be a good idea until you announce it.