It was working in r56669, but in both 56855 and 56912 it is broken. The above URL is just an example, but it seems to happen with any flash content I try. I was using the ClickToFlash plugin, but I uninstalled it when debugging this problem so it is not the culprit.
Nightlies no longer contain WebKitPluginAgent and WebKitPluginHost.
*** Bug 36956 has been marked as a duplicate of this bug. ***
Mark Rowe fixed this several days ago.