I just had the weirdest mental image... Laura Bailey as Kyouko, using a darker variant of the voice she uses for Maka Albarn.
Also, Vic Micnogna as Kyubey. *is shot a lot*
In all seriousness, here's my lineup for (part of) the main cast:
Michelle Ruff as Mami - Mami being a cool senior-type character, I'm picturing an odd blend of Yoko and Rukia's voices, depending on exactly how emotional Mami's getting at the moment.
Luci Christian as Madoka - has a cutesy, high-pitched voice, and is capable of a level of breathiness that gives off an air of innocence. Here's a scene from Princess Tutu where Duck isn't acting like a spazz.
Watch it and tell me I'm wrong.
Aforementioned Laura Bailey as Kyouko -
Just watch.
Trina Nishimura as Homura.
Ahem.
As for Kyubey and Sayaka... no bleeping idea. \
/ However, I think I'm a little too accustomed to the original to be able to listen to a dub. xD;
...
ACTUALLY, I LIED. Maxey Whitehead or Brittney Karbowski as Kyubey. Both are capable of innocent-sounding-yet-incredibly-creepy voices (Czeslaw Meyer, Selim Bradley), and let's face it - Kyubey sharing a voice with Alphonse Elric would be hilariously creepy, while sharing a voice with Selim Bradley would just be hilarious. Either actor could also bring out both Kyubey's cute and creepy side.
Still no idea who could be Sayaka though.