There are 5 mobile carriers that offer microSIM cards, but most of them are region-specific. Airtel and Vodafone will be your best bet for the widest coverage.
Airtel has plans that range from 1 day to 30 days. For example, a 7 day data plan of 150mb will cost you 49Rs (that's $US1… nope, not kidding).
For longer stays, there are 4 different 30 day plans, all at a good price, so it just depends on how much Internet you plan to use!
There is a stand in the airport for those hoping to be connected right away.
Vodafone is also a decent choice, with two 30-day plans of either 300mb (103Rs) or 1GB (251Rs).
Both carriers separate minutes/texts from 3G plans (Vodafone has an integrated plan that includes talk and text), so when purchasing your microSIM be sure to specify.
The most important thing to have when buying a microSIM is a copy of your passport and visa. Without them employees will refuse to help you. about 5 years ago