They usually announce the "next year's theme" in or near October of the preceding year.
They did that with the new "Celebrations" theme this year.
Expect the "Year of a Million Dreams" to continue thru Dec. 2008.
As always, exact dates are known only to (and subject to change by) Disney.
As to the Castle Suite(s),
I'm not sure if guests will continue to win overnight stays there as part of the new theme or exactly how they will use the Suite(s) in the coming years.
(Lots of rumors.)
Stay tuned.