A2A, thank you for all the helpful information.

Two more questions:
1. If I have to choose between Airtel and Vodaphone, which would have better data access coverage? (I'll probably carry an older Blackberry, 2G?. I'll be in CCU, DEL, Lucknow).

2. I understand that a photocopy of my passport will be retained by the shop that activates a SIM for me. Should I worry about the passport information being misused? I do have an old (circa 1990s) driver license, but I doubt any store would accept its photocopy in lieu of the passport.