Which environment? Flex 3 or Flex 4?
Christopher Coenraets did a simple example a long time back http://coenraets.org/blog/2007/05/new-mdi-style-trader-desktop-with-flex-and-apollo/
I've used a variant of this in Flex 4 after some minor cleanup and partial spark-ification.
There's also the flexlib project at http://code.google.com/p/flexlib/ that has MDIContainer and a bunch of MDI related stuff. Not sparkified yet, afaik.
it is not working .give any other link