I very highly doubt they're going to come out with it in the year of '08 considering the fifth is coming out in '07. It wouldn't make sense..but if they do then the movie won't be good at all probably. I might, but it'll seem very rushed and that's not cool..I felt like GoF was very rushed and should have been done in two parts like they originally planned.
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I just tried looking up some info on the IMDB site through google search on it and it looks like that may be right... But it's possible that they're doing what they did with PotC II and III by filming them at the same time and then releasing the next a year from the one that comes out first.
Harry Potter and the Half Blood Prince (that website I rely on more considering it's ment for movies and such..imdb is great) still has Dan, Emma and Rupert for the three main characters..it's a surprise but what a wonder makeup can do! I hope the Phelps twins are still in it..I know they'll be in the fifth being they don't show up much but they're twenty now (They turn 21 next February) so who knows.