To rename a virtual disk, first remove it from any virtual machine that contains the disk (choose VM > Settings > <virtualdisk>, then click Remove).
Then use the following:
vmware-vdiskmanager -n myDisk.vmdk myNewDisk.vmdk
Very useful for cleaning up a virtual disks created by using the importer, which creates really useful names like W2KR2_WSS3_CFirth (2).vmdk, where the (2) indicates that the drive was D drive on the MS Virtual Server image.