This is trivial I know, but I could not find this anywhere.
Using trial and error I figured out that in order to silence a blackberry you have to roll the trackwheel. Pressing the red "ignore" button while the phone is ringing sends the call right away to voice mail, and the caller no doubt feels snubbed. Whereas if you roll the trackwheel it quiets the phone but still "rings" for the caller.
EDIT- I called it a scroll wheel (sorry) its supposed to be called the Trackwheel. Fixed.