Read more about this here: http://www.techtree.com/India/News/Coming_Mobile_Number_Portability/551-84416-613.html
The moment this is implemented, I am most surely going to change from AirTel to BSNL. The only reason I am not moving away from AirTel right now is because it is a pain to inform everybody about the new number. Mobile-Numbers are right now used as vendor-lock-in mechanisms by service providers. Once they become portable, I will have a choice!