This question was posted in response to the following article: http://help.adobe.com/en_US/flex/using/WS02f7d8d4857b1677-165a04e1126951a2d98-7fd3.html
If you are going from 4.5.1 to 4.6 things have changed. Suddenly everything renders differently and the selectable property no long works. Set the skinClass property to "spark.skins.mobile.TextAreaSkin" and the world becomes less optimized but normal.
This is mentioned in the documentation.