Craig Hockenberry explains how in-app browsers can eavesdrop on your typing, and therefore potentially capture usernames and passwords. It is worth reading, but here is the bottom line:
Another goal of this essay is to increase user awareness of the potential dangers of using an in-app browser. You should never enter any private information while you’re using an app that’s not Safari.
Jason Snell provides a little more background to the story on Six Colors.