They're releasing them every 1.5 years. It's just easier for them schedule-wise since kids under 18 have very strict filming schedules due to labor laws in Britain. As of now, Daniel Radcliffe is only about a year older than his character so at the rate they're going, he won't be more than two years older by the time they complete the series. That's certainly acceptable.

They are indeed already filming movie five with an intended release in June '07. HBP will be Thanksgiving '08 and the final one will likely be Summer '10. It would be great if they could eek it out by Thanksgiving '09 since they'll all be over 18 by then and can work adult hours. Just 'cause I like HP movies over winter holidays vs in the massive push of summer blockbusters.

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Oldman has FINALLY been locked into returning for OOTP after threatening to walk for lack of a contract. That's good news.