The containers from the mx namespace subclass mx.core.Container which does not have a layout property. In the previous halo component architecture, layouts were programmed inside of the actual component.
The Spark components have abstracted the layout into a LayoutBase class. The mx.containers.Form class is an mx.core.Container decendent and thus has no layout property. For containers to have the layout property they usually subclass SkinnableContainer or GroupBase at the least.