Buy an ST. I can't see Ford bringing an RS here. As stated in this thread and many others, Americans in general (not the folks on this board!) just see a Focus as a small economy car, not something to drop north of $35k on. For that money most Americans would look at a Camaro or Mustang or, if they prefer European, perhaps an A3 or a 135i.
Buy an ST. Enjoy the hell out of it. If an RS eventually comes state side, you can probably sell the ST without too bad a hit.