On 4.0.4 ICS update Samsung changed Galaxy S2 screen capture key combination. The previous combo was Volume down (-) key and power button. The new method is also a two key combination. I feel its a bit faster than the previous combo.
Follow the simple steps to take screen shot in Galaxy S2 Stock 4.0.4 ROM.
1. Go to the screen you want to capture.
2. Simultaneously press and hold the Home Button + Power Button for 2 seconds.
That's it. You will hear a capture sound and a Toast message. To get the captured image go to /sdcard/Pictures/Screenshots.