An application dosn't need your username or password, since you can't use it without an account anyway. So you would be dumb if you entered it in a fb application, nothing to be alarmed about, just don't enter your password on anything not official. (Facebook has "approved" applications that costs the user $250 to have them check and approve, this is starting when the new facebook comes out)
Doesn't everyplace have the potential to have a virus?