As shown below you can find ringtones in the "/private/var/stash/Ringones*" directory of your iPhone.

Different reports have said that the .m4r format is really an MP4 file and some have said AAC and some also say "it's the same as the M4A" format.  Whatever the case is, one easy way is to install the silent theme from Cydia and copy that into your ringtones directory.

/private/var/stash/Ringtones.sh7cLj root# ls
Alarm.m4r        Crickets.m4r    Old\ Car\ Horn.m4r  Sonar.m4r
Ascending.m4r    Digital.m4r     Old\ Phone.m4r      Strum.m4r
Bark.m4r         Doorbell.m4r    Piano\ Riff.m4r     Timba.m4r
Bell\ Tower.m4r  Duck.m4r        Pinball.m4r         Time\ Passing.m4r
Blues.m4r        Harp.m4r        Robot.m4r           Trill.m4r
Boing.m4r        Motorcycle.m4r  Sci-Fi.m4r          Xylophone.m4r

I called mine "Silent.m4r" and it shows up under your ringtones, and I then set it under specific numbers I get constant calls from so it does not disturb me anymore.  This is yet another reason why it's great to jailbreak your iPhone since Apple doesn't have any Blackberry like settings to control different volume sources at all.