Click Edit>Camera Raw Preferences
Under Jpeg and Tiff Handling, uncheck what you don't want ACR to handle.
I'm running CS4, Bridge and LightRoom 2.3. What I would like is to have the choice when I open a tiff or jpeg to open it in ACR or Photoshop. However, any combination of preferences I have made either forces the tiffs and jpegs into ACR, or else won't let me open them in ACR. Is there a key combination or other trick so that I have the choice with any image I want to open?