biologists' investigation of DNA "has shown, by the almost unbelievable complexity of the arrangements which are needed to produce (life), that intelligence must have been involved"
Apparently, Flew's reasons for changing is mind about the possible existence of God is similar to the Intelligent Design (ID) movement's which argues that the complexity of nature requires an intelligence behind it. You can read the whole story here.
You might also like to read the press release from the Institute for Metascientific Research who ran the symposium at which Flew revealed his change of mind. Another interesting site on Flew is here where you can read past writings and comments regarding Flew.
All in all, quite a remarkable event.