I spoke to a very nice and knowledgeable person from VMware support yesterday and she confirmed what I thought I already knew. vSphere can't see the LUN due to an APD issue:
/var/log # cat /var/log/vmkernel.log | grep naa.600000e00d000000000127c500040000
2013-05-23T18:38:20.336Z cpu6:10283)ScsiDeviceIO: 2316: Cmd(0x4124007885c0) 0x1a, CmdSN 0x13b92 from world 0 to dev "naa.600000e00d000000000127c500040000" failed H:0x5 D:0x0 P:0x0 Possible sense data: 0x0 0x0 0x0.
2013-05-23T18:39:00.342Z cpu7:10363)ScsiDeviceIO: 2316: Cmd(0x4124007885c0) 0x28, CmdSN 0x13b93 from world 0 to dev "naa.600000e00d000000000127c500040000" failed H:0x5 D:0x0 P:0x0 Possible sense data: 0x0 0x0 0x0.
2013-05-23T18:39:00.342Z cpu6:93551)Partition: 414: Failed read for "naa.600000e00d000000000127c500040000": I/O error
2013-05-23T18:39:00.342Z cpu6:93551)Partition: 1020: Failed to read protective mbr on "naa.600000e00d000000000127c500040000" : I/O error
2013-05-23T18:39:00.342Z cpu6:93551)WARNING: Partition: 1129: Partition table read from device naa.600000e00d000000000127c500040000 failed: I/O error
I'll be getting in touch with Oracle later who support our FC switches to see if they can shed any light from a zoning point of view. I'm suspecting though that the data is already gone and I'll have to bite the bullet and format that datastore and present it again.