Don't be very happy Lord. Airtel is testing a feature knows as "Speed on Demand" DSLAM by DSLAM, therefore for couple of days or may be 4-5 , customers lying on DSLAM being tested will get 2.5 Mbps speed.
The same thing was going on with Tata Indicom Here in Bombay Few months back he was on 128 kbps plan & was getting 500+ kbps download speed...
But they reverted back to normal speed after few weeks.
The reason for this was they were upgrading to 2 mbps line.........
So u lucky guys download whatever u want if u have unlimited plans till the stars are shining on you...........