GSM cell phone unlocking

I'm talking about service provider lock, your cell phone company locks your
handset so that if you insert another carrier's SIM card the phone will not
operate. You must ask them to tell you how to unlock it ("the unlock code"),
who may or may not be willing to do so. I know there are other types of
locks, but this is definitely the most common one.

I searched the web and there are a bunch of third parties offering "unlocking
service" for $25 or so. Nokia phones are rather easy to unlock, you can DIY
freely for many of its models. But unlocking Motorola phones are much harder,
it seems you have to use a pay service (who has a "clip" to crack the phone's
internal software) if your service provider doesn't give you the unlock code.

I'm interested in the technical details of the locking/unlocking, how does it
work? To my knowledge the mechanism varies for different phones, some
simply calculate the unlock code based on IMEI + MNC + MCC, some generate
the code randomly and only the original carrier knows it. In latter case the
only way is to use the "clip" and data cable to change the handset's firmware
to bypass the locking function.

There is very little tech info I could find on this topic. I wonder if someone
who has experience on hacking those locks can shed some light on it.
Especially I'd like to know if there is DIY method to unlock Motorola
V300/V400/V600 because I'm thinking about buying one.