Yeah I saw Brad Bird tweeting about this! Pretty exciting, I hope we hear more soon of the other people (cast and other crew etc).
We're probably not going to hear news on the casting till at least by next year if not later (probably around D23, by then the story script would be long finished and filming would start) unless Bird posts it on sites and Yahoo gets a wind of it (since he has a minor habit of revealing things a little early), the biggest problems regarding the casting is Dash (do they dare age him to near adult and try to bring Spencer Fox back (they could bring him back and have him voice a new character if they can't find anyone) or go with the current voice actor in Raymond Ocha (who's likely going to start his teenage years voicing a pre-teen as he already did with Arlo) and Mirage (since her VA Elizabeth Pena died last year, she'll probably get a new VA or get axed completely like Doc Hudson did in Cars 2 (if they have to cast Sofia Vegara as Mirage, I'm willing to take a gamble, although I'm not sure if she could off the same femme fatale vibe that Pena did with Mirage, as Vegara is mostly known for comedy), Jack-Jack would also be a minor issue depending on how Bird wants to take the sequel (keep the family the same age as in the 1st film, or age the kids to adults to the point J-J needs a voice), I can almost bank on Mr. Incredible's, Elastigirl's and Violet's VAs returning regardless how their characters look in the sequel (although Sarah Vowell might have to change her voice pitch a bit to make Violet sound adult-like, that's assuming he ages the family).
But with the way Bird is likely heading with the info and how quick he got the script almost finished within the year (3/4ths in a year of starting is a very good pace) he wants to keep the family looking familiar to those who grew up with them while introducing new characters as well as bringing them in for newer fans (Disney Infinity is kind of already doing that in a way.)
I just hope Violet gets very minimal changes should they age her and the family, but I'm fairly content with whatever direction Bird goes with her character development (as long as he keeps her how she was in the 1st film for the most part).
EDIT: Bird posted on his Twitter account that the sequel will put a focus on Jack-Jack, setting is 5 years later with Violet and Dash as the new main supers defending Metroville with Bob and Helen nearing retirement raising a 6 year old rebellious J-J who's losing control of his powers, Violet is in collage while Dash is in Violet's old school from the 1st film, the kids (aside from J-J) will remain virtually the same appearance-wise from the 1st film with mostly minor additions (mostly likely making them taller while aging their facial appearances to make them look older but make them feel familiar to the fans who grew up with them.)