Paper Curl Shadow

[steve.ledger]

Create a black rectangle, convert it to editable shapes, use the shape tool to pull up the bottom line to make a slight arc.
Use a combination of the feather tool and linear transparency tool then position it behind your main object to suit.

[iagman]

jpdp,
I feel curl shadows are more effective when offset and adding to the illusion
that the light source is not directly overhead. Adding a bit of shading to
the paper can also help.