This is a confirmed root solution (without changing the kernel or using ODIN) for Galaxy S2 ICS stock ROM. You just need to enter CWM (Clockworkmod recovery) and apply some updates.
Model: Samsung Galaxy S2 I9100
ROM: Android 4.0.3 stock ROM XXLPQ
1. download the file below
2. unpack it and copy the content to your SD card
3. enter CWM - upArrow, Home Key, and power button pressed simultaneously
4. search for "Install zip from sdcard"
5. choose "CWM.zip"
6. touch clockworkmod recovery will run
7. search for "Install zip from sdcard"
8. search for SU-Busybox-Installer.zip and apply the update
Guide for UNROOTING is available inside the package (in short: apply the SU-Uninstaller-Signed.zip update)
Tested on stock 4.0.3 LPH & LPQ rom.