Disney is hiring Michael Arndt to write a draft of the script, said a person familiar with the movie who was not authorized to talk about it publicly. Arndt brings some heat--he's of course the writer of "Toy Story 3," for which he was nominated for an Oscar, and also won an Academy Award for writing "Little Miss Sunshine" back in 2007.
The initial "Phineas and Ferb" script has been written by show creators Swampy Marsh and Dan Povenmire, who have spent recent months working on that script and continue to work on the show. Unlike "Toy Story"--and the cable series itself--the "Phineas and Ferb" movie will be a mix of live action and animation. A Disney spokesman could not immediately be reached for comment.
Looks like July 2013 will be the release period for the movie. By then, I'm sure Perry the Platypus will be so popular in his own right, he will have his own spin-off! :)