The speed doubling is only for those who were on the Home995 or some plan which is hour-based. The new speed also has the same hour-based limitation. I confirmed it with an airtel customer care lady.
Actually, I've been hearing about speed increase with the Home999 guys as well. But since only a couple of guys were getting these, it is safe to assume that it was either an error on their end, or they are testing out the network capacity to see if they can allow unlimited 512kbps plan
So ppl on Home999, including me, can only hope airtel's kindness extends onto the unlimited plans as well.