I am installing a new copy of debian squeeze. I decided to use an encrypted Lvm this time. I noticed that is taking a really really long time to format the drive. It says it is erasing partition #5 which usually swap. Why is it wiping the partition?
Because the swap partition contains data that was pushed out of memory. As such, it could contain unencrypted data, representing a security risk. If you're using on-disk encryption, presumably you are concerned about the machine falling into unauthorised hands. Just as you don't want the data partitions to be readable by unauthorised users, you don't want the swap partition readable, as that could reveal data also.