Xperia X10 :: Cannot Hide Onscreen Keypad By Pressing Back Key

On the instruction, it clearly says I could hide the onscreen keypad by pressing the "back" key, this also works on my friend's phone. This doesn't work on mine, whether messaging or composing an email, when I press the "back" key it takes me back previous screen. It is really annoying as the screen is small and when you are typing a long-ish text you having to look at it through 1/3 of the screen! I reset the phone twice now and also downloaded the sony ericsson update launcher and have the latest software on the phone now, but this keypad problem still remain.