CID Lock V SIM Lock
CID Lock - means the device is locked to the ROM either by your carrier, if you have a GSM company provided/branded handset (e.g O2, T-Mobile et.c)
SIM Lock - means the service provider will not let you use the handset with another SIM card i.e if SIM Locked, you may not be able to use another service provider SIM on your handset.
I believe O2 now ship their phones SIM UNLOCKED (which is a great thing), therefore you can insert a T-Mobile, Vodafone, Orange et.c SIM into the handset they provided and it would work, however the handsets are CID Locked, so you cannot flash the ROM to another...unless of course you want to try pof's method.
......my 2 cents
There is mothing made by man that cannot be reproduced.