Quick Description: On Windows 2008 R2, even after installing the web server role and registering the .net framework with IIS, reporting services is greyed out as an option when attempting to install it to an existing instance.
A while back I wrote an article on installing Reporting Services on an existing SQL 2005 instance. These steps work for Windows 2003. However, if you should for some reason want to install SQL 2005 SP3 on Windows 2008 R2 (it's not the most common configuration, but maybe your licensing only allows up to 2005), these steps won't get you to the point where SQL 2005 will allow you to install reporting services. In my case, after following the steps in the original article (substituting adding the role in Server Manager, instead of adding IIS in add/remove windows components), and registering .net with IIS, Reporting Services was still greyed out as an option in setup.
I fumbled my way through this one before finding this article that Microsoft published on the subject. It's messy, but it works. It also probably demonstrates that SQL 2005 isn't really meant to be installed on Windows 2008 R2.