I have tried $(".ui-page-active textarea").keyup(); it doesn't appear to do anything. From what I can see the mentioned pull request has been rejected and there haven't been many comments in the issue log further.
In all browsers I have tested in (Firefox, Chrome, Iphone 4) the autogrow only works once you start typing. A royal pain if you only want to view the text as you have to type something for it to grow.
I know this topic is over a year old, but since it is one of the first google search results for "jquery mobile textarea autogrow" I guess its ok to post an answer:
The autogrow-functionality is called whenever the textarea receives a "keyup"-event. Textareas will only grow, but not shrink. To manually let the textarea grow simply fire the keyup-event on that textarea. example: