Can't say I've ever seen this on my Joe Pye Weed but it's amazing all the same.
Proof that Butterfly weed, Asclepius tuberosa, actually does get butterflies on it. I believe this is more common in the southern states though. By the time it blooms up north where I am it's hit or miss. To many other options and then there's the question of what host plants are present.
Also see The Wild Garden (really): butterflies and milkweeds
This was another great video, it's a shame they disabled embedding. It really shows a rich diversity of pretty butterflies. It's the kind of video that could really sell some gardeners on buying the plant.