Many people are already going with 'nobody'. Diversity is a key factor to immersion, and a skillful, never heard of voice actor is better than two celebrities which you instantly connect to something else, not a TES game...
That said I liked Jen in ME, even though she wasn't brutal enough when the situation would have implied it. The Witcher had wonderful voice actors, and I think most of the funny stuff that rang to your ear was due to translation problems. Also, a good writing in dialogues saves a lot, even some bad acting.
Who I'd like to see is Michael Wincott. The guy with da Voice. (The Crow, Strange Days etc)
Michael Wincott rocks, nuff said. It would be cool to have some "genre-specific" actors involved, like Sean Bean (done!), Christopher Lee, Jennifer Hale, or even a Viggo Mortenson. ABSOLUTELY NO ORLANDO BLOOM! He's the only British guy I know who sounds like he's faking the accent! lol. Maybe Antonio Banderas (Zorro), he has a really definitive latino accent, which has been missing from TES, mostly.
"Gun" had good work from Brad Dourif, Kris Kristofferson (RIP), Lance Henrikson, and Thomas Jane. I think celeb voices are fun, but they don't necessarily have to be big names.
I have no prob with celebrity voices, it's actually kind of cool. I don't think you can blame the lack of dialogue diversity on the expense of hiring celeb voices, either. The dialogue must be programmed anyway, the celeb thing is just icing. If it comes down to celeb voices vs. better gameplay? Gameplay all the way. But a lot of games have budgets as big or bigger than a lot of movies. I'd say follow this formula: Hire celeb voices ONLY if you plan on spending the money to do that AND make an innovative game. Do not sacrifice one for the other.