Most Flex development; as faras I know; is done by building business application; so I imagine there is not a big demand for integrating Flex with ad networks.
If you have a budget, contact us and we can review the feasibility of building a component for you that integrates with OpenX:
you don't technically have to hard code/stream adverts into the flex app
Push comes to shove, you could just set aside an iframe component:
it essentially just displays HTML objects, meaning you can have it act the same way as you'd display an ad on any website, might be worth considering as it would most likely be the simplest option
You could do that too, but honestly, for ads there is a simple solution: just put the ads in HTML next to the Flash viewer (match the background colors and it should appear seamless). I would think this should be fine in most if not all cases, although I can't say I have vast experiences in this type of development work. :)