we've already developed this as a premium soft-service & have deployed it at several telcos already. it provides 2 lists: allow & deny & 3 modes allow all, allowed list only & block all. and the telcos provide this service @ as low as INR30.00 per month ( $1)!!!
there are several s/w available on symbian which can do this already... i think that the overhead is just as much... adding numbers to the list.
apart from that, here in India there is a national DND list, as instructed by the Supreme Court & TRAI... you can get ur number listed on it (toll free ofcourse :P) - no unsolicited calls! and if you do, you know who is in trouble...