I jumped onto an airtel DATASHARE plan as soon as it came out and immediately realized that their support was horrible. Most of the service staff did not that there was a plan. And escalations simply wasted money on their customer care charges.

At one point the customer service told me that the plan was for only data cards. I promptly got a datacard and lo and behold it did work. The plan also works on any 3G phone. Here's how.

Go to pay.airtel.com and choose your circle recharge.  See if your circle provides a DATASHARE plan. In Bangalore it costs 998INR

Now send the following as SMS to 121
DATASHARE ADD Your_Phone_No

Eg: DATASHARE ADD 9876543210

If you get an error that the number is already a child in the system. It indicates that you or someone may have added your phone on their DATASHARE plan. Simply ask them to send the following as SMS to 121

DATASHARE DELETE Your_Phone_No

Airtel also has a website that has rarely worked for me, it apparently lets you manage data sharing.
Http://airtel.in/datashare

Enjoy your plan!

If you have any tips please drop them in as comments.