Worgens can transform to Human and back anytime in WoW Cataclysm!
Type /transform to transform between Worgen and Human form.
At level 10, Worgens get the "two forms" skill (which can be put on the actionbar) to switch between Human and Worgen form. But you can type /transform anytime after you gain the wolf form, even before you get that skill.
After level 10, click the "Two Forms" skill icon to avoid typing.