To take a screenshot in Android 4.1.x (Jelly Bean) the same button combination from Ice Cream Sandwich (ICS) works:
Hold Volume Down & Power buttons for roughly a second.
You should know when the screenshot has been taken, the screen will flash white and a translation effect of the screen will take place. The screenshot should be saved in a folder /Pictures/Screenshots on your phones internal SD card, it’ll show up in the Gallery app under a “Screenshots” folder.
Taking a screenshot will also causes a notification (similar to in ICS), which display slightly different to those found in ICS, they now show a cropped preview of the screenshot you’ve just taken.
This process works in Cyanongenmod 10, running on my Samsung Galaxy S (I9000) and should translate to all other phones and ROM’s. If it doesn’t be sure to leave a comment below.