I am assuming there IS gonna be a US release, as it has yet to be denied, and the Famitsu interview mentioned KH's WORLDWIDE popularity.
I predict around July - September. That should be enough time.
I would say september because March 29, 2007 is about 5 years ago that they released the very first version of the original Kingdom Hearts(according to Wikipedia it is March 28, 2002) in Japan, so my guess for a release of Kingdom Hearts 2 Final Mix + in America would be in mid September because 5 years ago was when the American version of the original Kingdom Hearts came out(Wikipedia says Sept. 17, 2002) and the same may go for Europe(Wikipedia says European version was Nov. 15, 2002 so the KH2FM+ for Europe would be in mid November).
The theory being that Square-Enix may spend 2007 celebrating the 5th anniversiry of Kingdom Hearts, and Square-Enix will probably want to release something Kingdom Hearts related in 2007, especally if there are thinking of combing that with annoucements of new Kingdom Hearts projects.