Well, I saw a confirmed report today on TORn (theonering.net) that put the extended edition at 4 hours 15 minutes. Not quite five hours, but still a nice sized movie to enjoy.
While nothing is confirmed, there's some speculation throughout the "Return of the King Movie Guide" on what may or may not be in the extended edition of RoTK which can be found on this site:
http://www.glyphweb.com/arda/
Plus, lots of cool and insightful information for anyone who wants to know everything about Tolkien they can.