Having a "back to top" button shouldn't cause any usability problems in and of itself, so its presence only aids usability. Adding it is a no-brainer.
It's good practice for every long page, really (even on non-mobile sites). The only downside is having to figure out how to incorporate it into the design - but that's not generally a significant issue.
EDIT: I should note that Nielsen's objections to multiple methods of moving around in the page don't hold here, in my opinion. Mobile browsers typically don't have efficient methods of scrolling to the top of a long page, and further I don't have his issue with there being multiple methods of accomplishing a task.