Flex 4 is pretty interesting -- they separated layout from the core of container components. So for example in XAML you have Grid, Canvas, Stack, etc. In Flex 4 you just have Group, which then has a layout property -- there's a few basic ones like basic(canvas) vertical, horizontal, and tile, but you can easily code your own... and then switch them on the fly.

Flex is basically a really damn good system for UI coding and design, but the ActionScript that backs it up is pretty weak compared to C#. If I could mix C# with MXML I would be in coding heaven...