It can be a bit confusing. In the past we have tended to call the free edition of Visual Studio provided in our installer a "Ruby language edition". However, now that we are also developing Amethyst for Flex, that description no longer fits the bill.
The good news is that the Shell Edition can be used to install both Amethyst and Ruby In Steel alongside one another. The Visual Studio Shell will also co-exist with other VS installations. More information can be found here: http://msdn.microsoft.com/en-us/vsx2008/products/bb933751.aspx