How To: Integrate with CanCan for roles management

Here is a detailed guide about how to implement Devise with CanCan:

http://www.tonyamoyal.com/2010/07/28/rails-authentication-with-devise-and-cancan-customizing-devise-controllers

You might also want to do it the easy way :

How To: Use CanCan + Devise, The easy way.