I've talked about this a lot, even produced some demos for presentation on the subject, yet there is one online you can see today.
The guys at Switch On the Code have put together a demo of using Flex Sliders to manipulate a Silverlight ball. They also posted a tutorial on how they were able to make it happen.
I did wonder why they didn't make reference to the ExternalInterface API (i.e. send the data back from Silverlight to Adobe Flex?) Anyway, it's yet another example of how the two runtimes can work with one another easily enough - provided you are open to the idea.
I'd classify these guys "Prototypers"