When creating any text using an iPad, you cannot select the text area in the HTML editor.

You can work around it by going to text mode (if you know about that), but that only works for an initial post – the editor that comes up in response to clicking the "edit" link on a post has a non-touchable text area both in HTML and text mode.

Note: Moodle 2.0 disables the HTML editor on iPad and has a patch pending to disable them on smaller iOS and Android devices. See MDL-23733