Most Playlists will disappear, Double EXP weekend will go, but 7 on the 7th will stay. I am told that Lone Wolves and Rumble pit will stay. Social Slayer and Team slayer will stay. These playlists will most likely become all DLC required.
So i would make general while you still can =)
Once Reach drops, Halo 3 will become the next H2, but XBL will still support it since it is for the Xbox "360".