If sales are good, I'd expect an announcement in a few months for the next set which would probably be released at the end of this year or the beginning of next year.
The new movie comes out June 30th, 2006. They did say in the chat a few months ago that the rest of the seasons would follow "faster than a speeding bullet"
I have no doubt it sold well, this show is too popular and has too much of a cult following not to have