[eluser]InsiteFX[/eluser]
It has something to do with the new CI 2.0, but for some reason it does not load all of the application core classes.
I would double check your __autoload and make sure it matches the one I posted for you, it is an update to Phil's __autoload.
I use HMVC and it uses SPL_Autoload so no need for me to use the __autoload in config.
InsiteFX