Here in Mumbai too on 21-9-2010, same problem, while I switch to mtnl default dns for some testing purpose & found lots of sites were unavailable, move to open dns again & problem solved
Had the same problem with my BSNL, changing the DNS helped a bit , but still some sites were not opening. Apart from DNS problem its something else too , i even typed the ip of my website still it was not opening. Meanwhile the solution that worked for me was using Opera - Turbo Mode ON.