The clear implication at the end of the film is that something of Data has remained and in the tie-in comics to the new 2009 reboot of Star Trek, we learn that when LaForge attempted to install Data's emotion chip into B-4, that was the final trigger needed for Data's memories and personality to override the crude personality of the B-4 android.
(Star Trek : Countdown 1 - "The Official Prequel to the Upcoming Motion Picture")
(Star Trek : Countdown 2 - "The Official Prequel to the Upcoming Motion Picture")