well, if your phone was faulty and you wanted a warranty repair - you wouldnt go to the retailer anyway would you?
Thats a repair under garauntee which is different.
A warranty is between you and the manufacturer. To unlock the phone without a code you will need to use hardware and nokia phones are incredibly good at recording what happens to the hardware and firmware. For instance, if you debrand your phone and it develops a fault you will have a hard time getting it fixed by nokia. The engineer will hook it up to his jig and see its not right - bang goes your warranty.
Same if you unlock it via a harware clip or box - the phone records what has happened. So, you need a repair - no luck, the engineer will tell you that the phone has recorded a hardware unlock that has not been authorised by nokia.
All, just so you know - there are only two places in the UK that do not belong to networks that have the ability to unlock phones and they are the two main nokia service hubs (one is infact a european hub), so finding out that a hardware unlock is not authorised is very simple.
Now, I am not trying to tell you all not to do it. Just trying to let you all know the implications about what this means.
Also, generating an unlock code is very difficult. The best you can hope for is an unlock application similar to the one doing the rounds for the N80. Of course, that program is illegal - as it means the firmware of either the phone, or nokia's official unlock apparatus has been reverse engineered (illegal in the UK). I have no idea if owning that software or using it is also illegal.
So, I dont have any problem giving a network £15 to unlock my phone