As the rest of the Telerik themes our Metro theme provides styles for several native Silverlight controls including TextBox. There should be no problem to apply Metro theme to a Button, TextBox, CheckBox etc. You may take a look at the following forum thread for further info.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!