It's hard to know what is multithreaded in a browser and what isn't. It seems while a video streams or progressively downloads, it does not affect page performance, so my guess it is.
Note I'm using Flash video, but it's really about video in general. Any other tips on what else is multithreaded (image loads?) is also helpful. I know JavaScript is not, and I thought Flash wasn't but I heard somewhere that it may be (or it could be done), but I think they were not well informed.