The new DBZ movie will be set during the 10 year gap between the defeat of Majin Buu and when Goku meets Uub at the World's Martial Arts Tournament. Here's an official quote from the script writer (Yūsuke Watanabe).
Strictly speaking, it’s between the comic’s “final chapter” and “just before that”. GT isn’t part of the original work, after all.
I’ve been given the honor of doing the script for the first Dragon Ball Z theatrical anime in 17 years. “Given the honor” really is the best way to say it. It’s a completely new, original work. Toriyama-sensei is truly great. I can go back to being a child. I think I’ll keep my hair dyed blond until [the movie] opens next spring.
It has also been reported that Akira Toriyama has been deeply involved with this project since the start. This movie will release in Japan on
March 30th 2013 with no plans on an English dub just yet.