Yes, there really was going to be a Buffy the Vampire Slayer cartoon. As a matter of fact, Joss Whedon was developing it for Fox Kids four years ago. Unfortunately, the project never got beyond a four-minute teaser clip. However, that clip has now surfaced online (EW was the first to pick up on it earlier today.) All you Buffy fans enjoy this, 'cause it's all you're gonna get:
The voice of Buffy was provided by Giselle Loren, who also filled Sarah Michelle Gellar's shoes in video game incarnations of the show.