Originally Posted by GUWonder

Taking a copy of your passport and visa, it's still possible to get a prepaid SIM card activated same day in some places in Delhi at least. There's always the "ask the ___ (e.g., driver) to get one for you (even if in his/her name)" possibility too if that will go faster.

In DEL, same day is still possible. (Email coming your way.)

[Sidenote: In Jammu & Kashmir (at least Srinagar) a prepaid SIM getting same-day activation is generally not possible for most all. The upside of a Kashmir-acquired SIM is that the Kashmir prepaid SIM will work in all of India while Indian prepaid SIMs will generally not work in Kashmir.]