1. The phone should jailbreaked.
2. you have to install OpenSSH
Connect with WinSCP or PuTTY into iPhone.
and, Run followed command
find / -iwholename *.lproj -not -iwholename *English.lproj -not -iwholename *en.lproj -not -iwholename *ko.lproj -exec rm -r {} \;

I just specify to except
         *English.lproj, *en.lproj    :: it is english
         *ko.lproj                            :: it is korean.

Actually, when I visit some country such as China, Korea. the bookbook is become to useless.
Such as
      friend_in_korea      011-82-11-111-1111
      it is correct number when I call from USA. but,
      If I am in Korea. I have to call 011-111-1111
      If I am in China, I have to call 00-82-11-111-1111
this kind of variations makes me so difficult to use the phone book...
Actually, I have put some multiple number in my phone book already.
If the smart phone is real smart.
this kind of issues can be resolved automatically.

When restore (or downgrade) firmware of iPhone 4 using SHSH Server.
iTunes will shows Error 1004 while restoring. and, iPhone will be stucked in Recover Mode.
Actually, the firmware has been restored correctly.
You just need to escape from the restore mode only.
Download RecBoot_v1.2.zip RecBoot_v1.1.zip , it is made by 'The Core-Dev Team'.
and, Run RecBoo.Exe

Personally, I like this software to access the file system of the Jailbreaked iPhone.
I recommend to get more information from http://i-funbox.com/
The reason why I am using this software
(1) There is no need to install any other tools
(2) Multi lanugage support (even file copy into iDevice)
* iFunBox v1.0.381.331 - ifunbox_en.zip

It is just my workflow to Jailbreak for iPhone 4
STEP 1. Recover and Clean Install
  (1) Run DFU mode
       - Run iTune
       - Press HOME and POWER button till screen is turn off
       - Release just POWER button till iTunes detect iPhone in Recover Mode
  (2) Restor Firmware using iTunes
  (3) Activate Cellular Network
  (4) Setup as New Device

Modding Mini SIM to Micro SIM for iPhon 4 (or iPad)
I have tested with China Mobile Mini-SIM Card to use in Unlocked iPhone 4.
The detail dimensions are descripted under photo.
1. SIM cutting for iPhone 4
    - Cut out around 2mm from left-end side (from the photo)

Sony VAIO Z Serise BIOS (sony_z_4044.zip )
          Version    R4044M3
          Modified to support
                          SLIC v2.1 - It makes possible to install Windows 7 OEM version
           How Flash
                           Need to DOS boot
                           > flashit R4044ADV.ROM /all

mkvtoolnix (mkvtoolnix-unicode-2.9.9-setup.exe)
        track editor such as track extract & insert
        * specially, usefull to remove an audio & subtitle track from MKV
AudioConverter (AudioConverter.exe)
        transcode audio in MKV file(s)

           (1) Data service will be activated within 24 hours. No need to wait till next month
           (2) You can deactivate the data service when you leave china
           (3) Cheapest way to get CMNET data service.

I'm using iPhone 3GS on AT&T Service in USA. I've tested with iPhone 2G also. 

STEP 1. Unlock iPhone
               You may can find so many unlock guide, so, I don't want to rewrite. 

Yesterday, new jailbreak software has been realease by Geohot. It's greaest new for every iPhone 3GS user.

                      You must jailbreak under WindowsXP 32bit or Windows 7 x86. It will not work under 64bit O/S or Vista
                      RC2 : The jailebreak software is working under XP, Vista, and Windows 7 (32bit, 64bit, and Internation)
                      RC2 : Sign issues were resolved. and now support VM_MEM kernnel fixing.
                      Please make your iPhone (step 2) before jailbreak to avoid some extra troube
                      When restore at DFU mode, remove your simcard to easy unlock.