I believe these re called FlowBox. If you google around people have implemented them in Flex 3; but I don't know if any Flex 4 versions exist.
this sounds like a great idea.
perhaps you can do it if you extend one of the existing containers.
I've never tried to do it so I cant help more at this point, but it might be something interesting to work on.
Have you looked at TileList?
TileList? That is something completely different from what i need...