As of today they have added this functionality:
It’s in the latest drop here:
http://unity.codeplex.com/SourceControl/changeset/view/33899
Discussion on it here:
http://unity.codeplex.com/Thread/View.aspx?ThreadId=66434
Example:
container.Resolve<IFoo>(new ParameterOverrides<Foo> { { "name", "bar" }, { "address", 42 } });"