Well, it’s pretty easy if you’ve already had fun flashing using Odin a decent number of times already. Off course This method can be used to unlock Samsung Galaxy S I9000 to all networks easy too.
Update: This version it's not official version from Samsung. All credits and thanks goes HERE on the official page of developer.
1 Download the latest version of “Android 4.0 ROM from here.
(Be careful while downloading since that page has downloads for all version of Galaxy S for which this rom is available. Download one meant for your phone, i9000.)
2 Transfer the rom file to your phone’s sdcard. Power off phone.
3 Press and hold these 3 buttons together: VolumeUP+Home+Power utill the Galaxy S logo shows up on screen. You’ll boot into CWM recovery soon enough. (In recovery, use Volume keys to scroll up and down and power key to select an option. Use back key to go back.
4 Do Wipe Data/Factory Reset - Do Wipe Cache Partition, Go to advanced and do Wipe Dalvik Cache
5 Select install zip from sdcard » choose zip from sdcard » browse through sdcard and select the rom file you downloaded to step 1
6 After flashing necessary stuff, phone will boot up into recovery again. You will notice it’s a different recovery, colored blue now, and this is what was flashed in above step to make the phone ready for installation.
7 Now, flash the rom file again. Select install zip from sdcard » choose zip from sdcard » browse through sdcard and select the rom file you downloaded to step 1.
8 Go back and select ‘reboot system now’ to restart the phone. Be patient while phone boots up, it may take between 5 to 15 mins for first boot (don't worry is just first time this).
Now you have ICS on your Galaxy S I9000
(tested method personally by me it's working 100%)
If you have questions please use comments contact or chat on-line.
ICS on Galaxy S i9000 [Overview]
Custom [ROM] ROOT