Hmm, seems like a simple idea.
I have just written a query about how to set a ringtone for "no caller id" [How can I change ringtone for "Unknown" callers with no caller ID?
], because my HTC Desire is currently using the "notification" tone as a ring tone, when I receive calls with no caller id.
So, perhaps you can add in an option to specify a ringtone for "no caller id" calls, as well as making it silent.
HTC Desire, Palm Treo 680, Zire 72, Zire 71, Palm Vx, Palm V, PalmPilot Pro
So much to do, so little time.