Check out this solution:  http://www.drake.org.uk/2012/07/os-x-mountain-lion-clean-install-gotcha-corestorage-encrypted-disk-issue/

Check out this solution: 

OS X Mountain Lion “clean” install gotcha: CoreStorage / Encrypted Disk issue

Basically, you need to run diskutil CoreStorage list from Terminal. Identify the logical CoreStorage volume from that list and note the UUID string.

Then type diskutil CoreStorage delete UUID where UUID is the string you identified.

You can then partition with Disk Utility and run a Mountain Lion install.