The problem is almost certainly that the partition type code for the partition has been changed. You can change it back with any of several utilities. I'm not very familiar with the command-line diskutil program, so I can't help you there. If it's an MBR disk, you could use fdisk to do the job. Unfortunately, I'm not very familiar with the OS X version of fdisk, but with the Linux version of the program, you'd launch fdisk, then use 'p' to view the partitions and identify the one you want to change, then use the 't' command to change the type code. Assuming this is an EFI System partition, the appropriate type code is 0xEF (you usually omit the "0x" part in disk utilities). You'd then type 'w' to write your changes to disk.

If it's a GPT disk, the appropriate code is C12A7328-F81F-11D2-BA4B-00A0C93EC93B, but most utilities (understandably) don't accept GUID codes like that directly. You could use my GPT fdisk to do the trick. It works much like Linux fdisk; launch the program, then use 'p' to view your partitions, then use 't' to change the type code. In GPT fdisk, that would be EF00. When finished, use 'w' to write your changes back to disk.