I normally do this manually, to get the most control over what's in each layer, but here's the info from the Illustrator online help section:
The Release to Layers command redistributes all of the items in a layer into individual layers and can build new objects in each layer based on the object's stacking order. You can use this feature to prepare files for Web animation work.
To release items to separate layers:
Select a layer or group in the Layers palette.
Do one of the following:
To release each item to a new layer, choose Release to Layers (Sequence) from the Layers palette menu.
To release items into layers and duplicate objects to create a cumulative sequence, choose Release to Layers (Build) from the Layers palette menu. The bottommost object appears in each of the new layers, and the topmost object only appears in the topmost layer. For example, assume Layer 1 contains a circle (bottommost object), a square, and a triangle (topmost object). This command creates three layers--one with a circle, square, and triangle; one with a circle and square; and one with just a circle. This is useful for creating cumulative animation sequences.