While Gumbo is still in active development, Flex 3.1 should be out this month and Flex 3.2 is planned for fall 2008. Gumbo will introduce new language features and different set of components that will make some existing Flex 3 components absolute. FXG is new document format and compiler will treat this format as external asset. New tags in FXG and MXML 2009 will be < Private >, < Library >, < Declarations >, < Definition >, < DesignLayer > and < Reparent >.
Instead of current http://www.adobe.com/2006/mxml namespace, Gumbo will introduce new language namespace http://ns.adobe.com/mxml/2009, but Gumbo will also recognize old namespace too. Some older tags like Binding, Script, Style and others will loose mx: prefix. But, built-in types like Array, Number etc will also loose it’s prefix.
Full details on MXML 2009 and FXG, but also if you wish to see it, here is example : Skinning a button in Flex 4 using FXG.