Good news for all those callers who want to retain their unique number to stay connected among their group. TRAI has come to rescue.
TRAI has recommended a country-wide implementation of Mobile Number Portability (MNP) policy which would be implemented by June 2009. However the policy also says that the period required to jump from one service provider to another should not be more than two hours, during which the customer will not be able to receive any incoming calls. Another caution that the prepaid callers will not be able to transfer the existing balance while switching lanes.

Well that we have to take it with pinch of salt.
Coming to adding gyan to this article, mobile number portability exists in countries like US, Australia, Singapore, HongKong and some European countries.