This series is scheduled for 6 books and the Internet Movie Database has the The Alchemyst movie listed as in developement for 2012. There is also an Alchemyst Game website narrated by Michael Scott which is a lot of fun. You can find it here.
Book 4 The Necromancer is due out May 25th 2010 and you can bet I will be in line to buy my hardcover and have it finished in one night. (I hope!)