Atrix: Turn the phone over. Slide your finger vertically across the reader. Turn your phone back over and commence use of phone.
Apple: Place your finger on the most used button on the phone, any direction you would like.
Edit: Fair point that you don't have to turn the phone over. I think it is still quite a jump forward in usability to incorporate it up front, embedded into a button that people constantly use on the device, and usable in any position.