Unlocking original iPhone 3g

Hi,

My roomate has a spare iPhone 3G that he bought and used on US AT&T network. We both moved to the UK and he is going to buy new 4G iPhone. I currently cannot justify the purchase price of the 4G (especially since O2 will probably want a couple of hundred pound deposit) but I though I could take the original 3G phone of his hands (my current Blackberry is about to die).

If I was to purchase the 3G from him I would like to do the following:

1. Upgrade to the OS4 for iPhone 3G.
2. Sim unlock the phone so I can use it on my O2 network

I am sure jailbraking has been covered a number of times, however if anyone has a step by step guide (as I would first like to upgrade the OS) that can help me do this it would be very helpful.

Kind regards,

As in a previous thread JailBreaking Taboo ?? we are not loud to discuss or tell you how to JB your iPhone as it violates the DMCA, but once it is JB we can talk about your iPhone and Apps you want to put on it.
We can discuss UNLOCKING, but that is all done through the current Network it is on.

Hope it helps some ??

Cheers