This has been confirmed by Leahy's office. Leahy is apparently a big comic book enthusiast, and actually served as an extra in the 1997 Batman installment: Batman & Robin.
The senator told the station he can't reveal the exact details of his role in the upcoming movie, but he did say he has landed a scene with its two stars, Christian Bale and Heath Ledger.
It's not particularly uncommon for senators and other Washington notables to accept movie roles when given the chance. Arizona Sen. John McCain made a brief cameo in the 2005 Summer blockbuster, Wedding Crashers. The cast of the 1993 political comedy Dave starred, among others, Sens. Chris Dodd, Tom Harkin, Howard Metzenbaum, Paul Simon and Alan Simpson, as well as Speaker Tip O'Neill.
Leahy previously had a voice role in Batman: The Animated Series playing...himself.