A nice clean news post for this since it appears we have the final cast now.
AWN was sent a press release by Toei that hasn't fully made the rounds yet. The primary major detail is that Joshua Seth is back as Tai, which will no doubt make many people very happy. Update- Joshua Seth has confirmed the press release's accuracy on his Twitter.
Full Cast list for the 'main' characters:
Tai- Joshua Seth
Agumon- Tom Fahn
Greymon- Kyle Hebert
Matt- Vic Mignogna
Gabumon and Garurumon- Kirk Thornton
Sora- Colleen O'Shaughnessey
Biyomon- Cherami Leigh
Birdramon- Melodee Spevack
Izzy- Mona Marshall
Tentomon and Kabuterimon- Jeff Nimoy
Joe- Robbie Daymond
Gomamon- R. Martin Klein
Mimi- Philece Sampler
Palmon- Anna Garduno
Togemon- Mari Devon
TK- Johnny Yong Bosch
Patamon- Laura Summer
Angemon- Dave Mallow
Kari- Tara Sands
Gatomon- Kate Higgins
Meiko- Cristina Vee
Meicoomon- Kate Higgins
Daigo- Doug Erholtz
Himekawa- Cherami Leigh
Dorothy Fahn mentions on her Twitter that she is voicing Tai and Kari's mom again. While not confirmed, it seems likely she will return as Lilymon in part 2 (and return as Rosemon she since voiced her in Data Squad.)
Todd Haberkorn confirmed in a podcast interview that he recorded for Digimon (about 4:35 in the interview), but as of yet we don't know who he is playing.
The AWN post also includes the few other not yet confirmed names that we already knew.
Lara Jill Miler (Kari in Adventure and 02) posted about her not being Kari in tri. (from the wording of her post it actually sounds like she's been aware for awhile she wasn't coming back, and chose not to let people know out of respect for letting the companies involve announce the new cast.
Michael Lindsay popped up on one of Jeff Nimoy's Facebook pages to mention it would have been nice to have at least been asked.
Michael Reisz commented on the casting of Vic Mignogna as Matt on his Twitter.
Shockingly most of the cast is back.
For the new cast members:
Kyle Hebert is new as Greymon in Adventure, but he's been Belphemon in Data Squad, and in Fusion he was Dorulumon, Ballistamon, and Greymon (and Beelzemon in the Data Squad game.) He is probably best known as the narrator and older Gohan in Dragon Ball Z.
Vic Mignogna is our new Matt. He was Kiriha/Christopher and MadLeomon in Fusion. As for his best known role, it'd probably be Edward Elric in both Fullmetal Alchemist series'.
Robbie Daymond is our new Joe. He is new to Digimon. His biggest role thus far is likely Mamoru/Tuxedo Mask in the new Sailor Moon dub.
Johnny Yong Bosch we already knew would be our new TK. He isn't new to Digimon, his biggest role likely being the bit part of Dracomon from Fusion. His biggest roles people would know would probably be Adam from Power Rangers, Vash from Trigun, or roles to numerous to list.
Tara Sands is our new Kari. She is new to Digimon. Her best known roles would likely be Mokuba Kaiba from Yu-Gi-Oh or Bulbasaur from Pokemon.
We knew Kate Higgins was Meicoomon, but it turns out she is also our new Gatomon. She was Miki in Digimon Data Squad, but is probably best known as Sakura in Naruto.
While Doug Erholtz isn't back as TK, we now know he is doing the voice of Daigo. Outside of TK, his best known role is likely Gin Ichimaru in Bleach.
And after they had mis-listed Himekawa's VA, they have now corrected it to Cherami Leigh who will also be Biyomon. She was Lopmon and Luka in Digimon Fusion, but is likely best known as Asuna from Sword Art Online, Lucy from Fairy Tail, Setsuna from the Negima franchise, and Sailor Venus from the new Sailor Moon dub.
Fathom put the short trailer on Youtube, thus far we have no trailer with the voices yet.
Prior tri. part 1 dub thread is here.
Information on theaters and ticketing for the Fathom event is here.
Thanks to Clay for pointing out the news.