According to ProWrestling.net, The Undertaker was spotted earlier at the airport in Buffalo, the host city of tonight's Old School edition of WWE Raw.
Additionally, WZ received an anonymous tip that The Undertaker and John Cena were spotted together in Pittsburgh yesterday, on their way to Buffalo.
UPDATE at 7pm EST: According to PWInsider.com, The Undertaker is indeed backstage at the First Niagara Center and has been included in the script for tonight's WWE Raw. Reports are saying that he will confront CM Punk and begin their match for Wrestlemania 29.