I made a test SL5 OOB project and tried to build under x64 platform but I get the warning:
"Assembly generation -- Referenced assembly 'CommonLanguageRuntimeLibrary' targets a different processor".
and an error:
Error 2 Could not load file or assembly 'file:///C:\work\TestSilverlightOOB\TestSilverlightOOB\obj\x64\Debug\TestSilverlightOOB.dll' or one of its dependencies. An attempt was made to load a program with an incorrect format. C:\Program Files (x86)\MSBuild\Microsoft\Silverlight\v5.0\Microsoft.Silverlight.Common.targets 214 9 TestSilverlightOOB
Same for WebHosted with RIA enabled.