Since a lot of people are complaining that you cant silence the ringtone, I found a little simple workaround that you can do in the meantime(nothing special, but simple)... is while the phone is in sleep mode, double click the home button.. it will bring up the ipod media player buttons(play, pause, last & next track) now slide the volume all the way to the left and now the ringer is silent and the phone will just vibrate... to reverse it do the same thing but of course move the slider all the way to the right.
I found out through a minute of trial and error that it used the volume from the media player and not the ringer, so this is what I feel is the easiest way to put your ringer on silent until they patch it up.