Palimpsest proposes a nice option to "encrypt the underlying device" of a new partition. When setting up such an encrypted partition, it is advised to fill it with random data to make cryptanalysis harder.

Please support filling newly created encrypted partitions with random data. Filling the unlocked device with zeros is enough, as the encryption will make them appear as (nearly) random data for an attacker that has only access to the locked volume.
This should probably be the default behaviour, but there could be an option to disable it as it may take a long time.