Thoughts from the abyss

# lvextend -L+5G /dev/vg0/whatever
  Extending logical volume whatever to 45.00 GB
  Logical volume whatever successfully resized

# cryptsetup resize /dev/mapper/whatever

# resize2fs /dev/mapper/whatever
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/mapper/whatever is mounted on /whatever; on-line resizing required
Performing an on-line resize of /dev/mapper/whatever to 11796351 (4k) blocks.
The filesystem on /dev/mapper/whatever is now 11796351 blocks long.

Voila! Your encrypted partition has been resized.