Since the link above is to FOX, I thought it might be educational to see what NBC has to say:
http://www.msnbc.msn.com/id/12137229/
It appears that a final decision on the successor has not been made yet and an NBC spokesperson wouldn't comment on the search.
ETA...watch the streaming video of her actual announcement.
As a side note, having been in broadcasting for years, I think it was pretty classy of NBC to allow her to handle it that way, to allow that much airtime and to allow Matt Lauer (who I think handled it with humor and class), to ask about future plans. It has been my experience in a few badly handled incidents in the past for management to simply escort the person who is leaving out of the building...which would have been a really bad move in this case.