On the Stubbles blog today, there's a warning from Frank Kleine about using the session.auto_start setting in your PHP installation and it preventing you from working with objects. As per the PHP manual:
If you do turn on session.auto_start then you cannot put objects into your sessions since the class definition has to be loaded before starting the session in order to recreate the objects in your session.
Frank points out that this also includes calling session_start before defining other classes to be used. They've come up with their own solution, though, using a base interface object for all of their classes. This object is included in each of the pages, eliminating the worry about having multiple objects and classes to mess with.