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I have been using Flex for a couple of years now and i still find new and interesting framework items. In comes IMXMLObject. Up until recently whenever i wanted to place a non visual component into mxml i was extending UIComponent to allow me to place these items in MXML, however among other things this meant that they would be included in the layout and come with a high footprint.
The solution seems to be to implement the IMXMLObject interface this will allow you to place non visual components into your mxml code without having to worry about layout or an unnecessary memory footprint.

usefull links
livedocs
smashedapples
techper