Comcast did it, so did AT&T in the US along with numerous other ISP's. It's the turn of Airtel to follow suit. As per the notice put up by Airtel they would be implementing an "Airtel' Fair Usage Policy" which gives them the power to cut down your bandwidth speed to half (at the same charges) after you cross a specified data transfer limit in your unlimited(They should now remove this word, isn't it?) data broadband limit.
The details of this awesome :-) plan are available here
The rest of the details are pretty sketchy here, not to mention since when this has been implemented, where we may check our usage, whats our usage etc. etc.
Strange that we do get automated calls and all kind of advertisement with our Airtel Bill, but Airtel found it quite unnecessary to inform us about this update from their end which would be hitting us directly. I found an online petition against this move from Airtel which can be accessible here http://afup.broadbandforum.in/
If you feel this issue is important to you, use the power of Social Media (twit, blog, social networking sites etc.) to spread the word around and make people aware of it, as looks like Airtel is not gonna do that for sure.