It seems as if Google is asking the AdSense publishers how they want to be paid and how much they want to be paid on a CPM basis for the AdSense rich media ads that are in beta now.
A thread at WebmasterWorld shows a member asking other members for advice on what to tell Google. He broke down the units as follows;
Expanding ads CPM: Floating ads CPM: Audio/Video ads CPM: Prestitial ads CPM: Intersitial ads CPM: Dedicated home page placement (no out of the box) CPM:
Forum discussion at WebmasterWorld.