Doctor Who has been officially confirmed as the longest-running sci-fi television show. The show debuted on November 23, 1963, and was revived in 2005 after 16 years off the screen. Nevertheless, it has clocked up 723 episodes featuring 10 different Doctors: William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davidson, Sylvester McCoy, Colin Baker, Paul McGann, Christopher Eccleston and David Tennant. American series Stargate SG-1 holds the record for longest-running consecutive sci-fi show.
PS: In unrelated TV news, congratulations to new father Shannon Noll. There's more at the ie blog.