On my Samsung Android phone (Android 2.3.5), and on my Android tablets (Android 4.0.3 and Android 3.1), this works fine - the textarea can be swiped up and down to get at the additional text.
It also works if I visit the page with a normal desktop browser, as I see scrollbars in a normal browser such as Chrome.
However, if I use an IOS device, the textarea will not scroll and it is impossible to get to the text that is not initially displayed. I checked this on my Iphone 4 (IOS 4.3) and my Ipod Touch (IOS 5.1.1)