According to Wikipedia, Rowling recently said that it wasn't impossible that she'd write an eight HP novel, but it wasn't hugely likely. I don't think anything's going to be written soon, because she's spending time with her family and getting Pottermore up and running.

I'd actually guess that any new HP books would focus on the kids. I don't think it'd be impossible to get at least one more book out of Harry himself. After all, Rowling said that Harry and Ron were both Aurors and Hermione had a very high-ranking job in the Ministry herself regarding equal rights and abolishing anti-Muggle laws. If a new threat came up, I could see them being in a position to fight.