Pikachu's voice actor is Ootani Ikue. This is her in an interview! Kawaii desu ne.

Host: Ms. Ootani Ikue?
Ikue: Yes!
Host: Nice to meet you.
Ikue: Nice to meet you!
Host: You dub Pikachu's voice? Can you say "Pikachu" for us?
Ikue: Pi-KA-chuuu...
That one didn't work so well...
Host: It DID! You're the real pikachu, huh? How cuuuute! Could you say it again?
Ikue: Pika PiKAAA
Host: Oh, that's it! You're real! You're voice even changed a little at the end...
Ikue: I'm sorry!
Host: Sorry? It's too late!

Any Pokemon fans out there? Comment with your thoughts!