I searched around my computer for about an hour for this… grr!!!
According to the documentation RS command line tools are installed as part of the reporting services install. They are installed to c:\Program Files\Microsoft SQL Server\100\Tools\Binn by default (unless you change the path in your installation).
When I checked this path they weren’t installed. After doing a Windows file search for rs.exe nothing turned up – I thought I must’ve missed something in the installation.
Turns out that instead they are installed to C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn. It must have something to do with it being a 32 bit utility even though the rest of the SQL Server and Reporting Services installation is a 64 bit install.