Remove the sim lock from the Samsung Galaxy S2 LTE SC-03D - Japan Docomo

When I did this I had stock 4.0.4 (SC03DOMMP5 / SC03DDCMMP5) flashed with ODIN from sammobile.com (you can do it with heimdall too I guess). *Very important* 

In order to sim unlock the SC-03D it is necessary to first flash a "special" radio. Flashing the radio is possible with a CWM based recovery. Get the latest version of CWM-recovery from this forum and flash it using ODIN/Heimdall:
http://android.slateblue.tk/viewtopic.php?f=20&t=73

Then get the radio to flash:
Official TMo ICS RADIO_NOTE.zip
(from this XDA thread:
http://forum.xda-developers.com/showpost.php?p=37564563)

This radio is from the Galaxy Note (SC-05D) so it made my phone very very slow. However, you need this radio because it has options to make the phone sim free. Options which the original SC-03D radio does not. As soon as you flash this new radio boot the phone, run the dialer application and dial:

*#9090#

it might be very slow but eventually you should get to the service menu.

To access more options press [menu] then [Input key]. Then into the text box type:

Q0   (Q Zero, not QOh)


It should bring you to another menu after a second or so. Select these options:

[1] DEBUG SCREEN

[8] PHONE CONTROL

[5] SIMULATION

[6] NETWORK LOCK

[3] PERSO SHA256 OFF

Wait for 30 seconds , press MENU then BACK.

Select:
[4] NW LOCK NV DATA INITIALLIZ

Wait 60 seconds. Reboot.

Congrats you're sim free. You should remain sim free no matter what radio/firmware/rom you flash.

partial credit to FAB: 
http://blogs.yahoo.co.jp/fab4_boys/26754215.html
And HomuHomu for the recovery.