It's not clear how this happened given that protesters or the army have been reported to be guarding the museum, but Zahi Hawass reports that looters have broken into the museum and have destroyed two mummies and ransacked the ticket office. See
Meanwhile, a fire is burning out of control at the nearby headquarters of President Mubarack's political party.
Obviously, I hope this does not get worse, but this episode probably reflects the fact that at least some ordinary Egyptians might just consider the Museum to be another outpost of a hated regime-- just as the many Iraqis considered the Iraq Museum to be. And the fact that it is so near to party headquarters probably does not help much.