One of my work colleagues is Jason Jones, who's also a ForeFront MVP. If you're interested in ForeFront Edge concepts, I strongly recommend that you check out Jason's blog Closer To The Edge.
Jason has advised myself and other team members this morning that although Unified Access Gateway (UAG) delegation to Exchange forms-based authentication technically worked pre-UAG SP1, it was not supported by the UAG product group. In fact, the GUI has now been amended to prevent it as shown from the figure below. Hat tip to Jason.