Note : I recently bought myself a Sony Xperia ION.
There are two ways to take screenshot with Sony Xperia series smartphones.
Method 1 (Applicable to Gingerbread) : If the phone has not been upgraded to the ICS (Ice Cream Sandwich) OS, hold down the POWER button and on the option screen is a TAKE SCREENSHOT option
Method 2 (Applicable to Android 4.0 ICS and above) : Hold down the VOLUME DOWN button and the POWER button.
Hope this helps :)