Web object scopes are working in WebApplicationContext, but they are not working in MvcApplicationContext as stated in the documentation. The reason is, that the support for web object scopes is given by spring.web.dll. Since the spring.web.mvc.dll is not depending on spring.web.dll, it is impossible to instantiate an MvcApplicationContext that can resolve session scoped objects. MvcApplicationContext actually behaves like a standard application context, which is using the class RootObjectDefinition whis doesn't know about the scope attribute. The WebApplicationContext instead instantiates RootWebObjectDefinition types, which know the scope.
We made a workaround to this issue and posted it on StackOverflow: http://stackoverflow.com/questions/6060386/spring-net-and-mvc3-on-iis7-session-scope-behaviour