I also have this problem (although got here from Mtn Lion).
How I got to the problem:
External 3TB disk had
500GB unencrypted time machine backup of my snow lep partition
500GB unencrypted time machine backup of my mountain lion partition
750GB unencrypted data files partition
750GB partition designated for encrypted time machine backup
500GB unassigned space
I started time machine making the backup and returned 8 hours later to find the backup 40% completed, stopped the backup, and desired to erase the partition and just live with unencrypted backups.
Disk utility shows the usual "whole drive" entry with four partitions under it, but the name of the "whole drive" has changed to be the same as the encrypted partition name, and selecting the encrypted partition states I can only split the partition; Selecting any unencrypted partition states I can only merge the partition with the one before it.
I rebooted to see if that affects anything, mounted and unmounted each partition. This is looking pretty bad.
I was able to fix my problem by:
1) booted from recovery partition
2) ran disk utility
3) clicked on the encrypted partition, clicked "unlock"
4) clicked on the encrypted partition, clicked erase tab,
changed format to Extended Journaled (not encrypted)
clicked erase button - Big Scare cause it briefly looked like I was erasing the whole disk.
When it was done, the disk name went back to what it was originally - great sign.
5) clicked on the whole disk, went to the partition tab
6) deleted the formerly encrypted partion just to be rid of any problems
7) added a partition in that space - extended os, journaled
8) rebooted into my OS, all the unencrypted partitions and contents were safe.
9) Set up Time Machine for a non-encrypted backup to the new partition and finally took a deep relief breath.