Not the first such game (Score 2, Interesting)

Kingdom Hearts, the big RPG collaboration between Square and Disney, has several pretty-well-known voice actors in it:

Haley Joel Osment - Sora
David Gallagher - Riku
Hayden Panettiere - Kairi
David Boreanaz - Leon
Mandy Moore - Aerith
Christy Carlson Romano - Yuffie
Lance Bass - Sephiroth
Sean Astin - Hercules
Dan Castellaneta - Genie

I guess this would be the first MMORPG with Hollywood actors, though. Doesn't seem like a real big deal to me.