I've just come hot-foot from TPMf with a hot tip from Pallette. Apparently the Japanese fans are currently saying that Pikachu uses a completely new move in a soon-to-be-aired episode. Which move? Iron Tail. It would make tactical sense for Ash to teach his Pika-buddy that move, as I'm sure that you will agree.
For now, this remains only a hot rumour rather than a confirmed spoiler. However, I think it is about time that Pikachu learnt a new move. Apparently the last time he used a totally new move was Double-Edge all the way back in the Kanto League.
I just want to add a little opinion of my own. IMHO, Iron Tail can only look good when used by a Pokémon with a really long tail. Pikachu has only that stiff, stubby little thunderbolt of a tail. A Raichu, with its' long streamer-like tail would look a lot better using the move. Could it be that learning this move is a sign of an impending Pika-evolution?
(Puts on flame-proof clothing )