The other week I “tweeted” that the Flex 4 Layout Mirroring spec posted at http://opensource.adobe.com/wiki/display/flexsdk/Layout+Mirroring. The spec menioned that layout mirroring will only be supported when targetting Flex 4.1:
This feature adds a property and a method to the existing IVisualElement interface. There probably aren’t many new implementations of this interface in the field however any examples that do exist would have to be recompiled to target Flex 4.1.
Today it looks like the Flex 4 downloads page on opensource.adobe.com site also has its first Flex 4.1 SDK posted — 188.8.131.5280.