I'm not sure if this is a bug or not, but after upgrading to rc2 I can't seem to access any pages that require the auth package (specifically the simpleauth). I got the following message: "ErrorException [ Error ]: Class 'Auth_Login_Simpleauth' not found"
Did you copy the auth.php config file to your app? If so you need to change the casing of the driver: it is now called "SimpleAuth" instead of "Simpleauth". Or if you were extending it it now needs to be called Auth_Login_SimpleAuth instead of Auth_Login_Simpleauth. (once loaded PHP doesn't care about a class's casing, but for the loading of core classes the upper/lowercase needs to be exact)
This wasn't consistent throughout and I decided to make it so, that might however break it for some people already configuring/extending the SimpleAuth class.
Upon updating from 1.4/master to 1.6/develop, I ran into this same issue. Apparently the change back to Auth_Login_Simpleauth from Auth_Login_SimpleAuth took place between 1.5/master and 1.6/develop. Specifically, it looks like WanWizard changed everything to lower case in a couple of Feb 15 commits entitled "fixed incorrect Auth class names".
According to the FuelPHP coding standards, and the way the autoloader works, Classnames should be ucfirst().
I am clueless to why Jelmer decided in the past do deviate from this rule, because it breaks extending if you want to place your extended class in a subdirectory (Auth_Login_Subdir_SimpleAuth was not accepted by the autoloader). His assumption that this change could break things were absolutely true.