Note: /dev/disk0 is the primary hard disk, formatted into three partitions of equal size, one of which is free space (and thus not shown); /dev/disk1 is the Intel-based iMac restore DVD.
$ diskutil list
/dev/disk0
#: type name size identifier
0: GUID_partition_scheme *152.7 GB disk0
1: EFI 200.0 MB disk0s1
2: Apple_HFS Macintosh HD 50.9 GB disk0s2
3: Apple_HFS Scratch 50.9 GB disk0s3
/dev/disk1
#: type name size identifier
0: Apple_partition_scheme *4.7 GB disk1
1: Apple_partition_map 31.5 KB disk1s1
2: Apple_Driver_ATAPI 4.0 KB disk1s2
3: Apple_HFS Mac OS X Install Disc 4.7 GB disk1s3