Eh well, I read somewhere (think it was Bulbapedia) that an animator confirmed that Misty will never return as a main character, but hopes to feature her is select episodes from time to time, kind of like they've done with her recently.
I also read that the first few episodes of Gen 5 have been created, and both Dawn AND Brock will be left behind. Team Rocket is still there though, from the looks of the concept sketches.
So yeah, no more Misty. Seeing as it's been that way for a while now, I don't see it changing. As for May and Dawn, they're okay. Never really got attached to them like Misty (I've always been a sucker for the "starting group", for lack of a better phrase), but they haven't been horrible or anything. I'll gladly put up with the next female lead that comes along.
Brock leaving upsets me a bit though. He's been there for so long, it'll be weird without him. Then again, I'm looking forward to a completely brand new cast, all built up from just Ash and Pikachu.