For those that are currently using the stable version of the CodeIgniter framework (1.x) and are wanting to know what they'll need to do to upgrade to the upcoming 2.0 release, Phil Sturgeon has just the guide you're looking for.

I got it all working in a few hours after LOTS of headbanging, mouse throwing and blaspheming. All the bugs I found along the way have since been fixed in the 2.0 branch by the Ellis developers or myself. While this does not mean everything is running perfectly [in PyroCMS] , at the time of writing all the worrying bugs are squashed so it should be safe for us to have a play.

He covers a few things like changes to the model class names, the use of plugins (or how you won't anymore), how they've dropped the validation class, that modular separation won't work out of the box and a few more. You can grab the latest version of the CodeIgniter 2.0 branch from the EllisLab bitbucket account (and contribute back if you find a bug!)