1. Turn on the phone.
2. Enter sim pin.
3. Perform a hard reset (hold down power and home buttons until phone turns off, and keep holding until Apple logo appears).
4. When the phone comes back on, slide unlock. It will not ask for a pin and will work. You will see searching after 5 seconds, and after about 30-60 seconds, the network will come online.

Now if someone came up with an app that performs a hard reset(not reboot...).

Cheers.

Update: does not work with very old sims.

THANK YOU SO MUCH. I didn't know this. I am traveling to Europe VERY soon and needed to use my local sim for over there which I bought. Much rather would like to pay 99 cents CAD a minute int calling rather then Rogers' roaming rates of 2.00+.
You made my day.

Absolutely. This made it for me too. I tested it with two SIM's; one local provider (Norway) and another German provider in roaming mode.
I guess this last step needs to be documented properly by yellowsn0w until they get another patch that fixes this issue?