Exchange 2010 introduced RBAC as a mechanism to manage access to administrative tasks at granular level which was not possible in previous versions of Exchange.
While you may know how to use RBAC to create custom roles that maps to job functions in your environment, one particular feature tends to get easily overlooked, mostly because it is least understood I believe. It is Unscoped Top Level Management Roles.
So, I wrote a blog post on it detailing what it is, and how to configure it. It went live few days ago at Hey, Scripting Guy! blog.
You can read complete article here – http://blogs.technet.com/b/heyscriptingguy/archive/2012/01/13/use-powershell-and-rbac-to-control-access-to-exchange-server-cmdlets.aspx
Originally posted at http://blogs.technet.com/bshukla