It's strange that this is only happening for new videos.
I had the same problem, and fixed it with John's suggestion of getting the new player.swf file. That fixed the player locking up problem.
However, this new video I just uploaded, for whatever reason, cannot display its thumbnail at all, and only shows a black screen. I've tried the thumbnail as both a JPG and PNG file. No other previous video files have this problem, only this new one.
I also uploaded the new yt.swf file as well, even though I didn't know if I should or not. It didn't seem to have any effect whatsoever.
Alex Rabe, if you're reading this, do you have any suggestions? It does seem like Longtail wants more advertising though, because they put that big block of banner in the new player.swf; ah well.