I just submitted a bug to Microsoft about the mess being created when you want to have both .NET RIA Services Tools for VS 2008 and Windows Azure Tools for VS2008.
When you have both Windows Azure tools and .NET RIA Services tools for Visual Studio 2008, the former will conflict the last.
In general, if you have installed only .NET RIA services tools for VS, a new type of project is added (affected one) - .NET RIA Services Library. It creates two projects - one Silverlight Library and one .NET Class library.
However, if you also have Windows Azure tools for VS installed, and want to create .NET RIA Serivces Library, the second project type will be Windows Azure Worker Role