In the following series of posts, I'll demonstrate how you can't put an un-thumbnailed video in the forum and expect it to play.
But first, a few examples of what does work.
Here's a video taken directly from w:c:video. Both of these examples play.
A video at 501px, not thumbnail. The instruction given was that this should align right, but since local customisation keeps the width of the thread to almost 500px, anyway, there's no real difference between |right and |center.
The fact that this video is breaking the boundary of the thread is entirely due to local customisation. Once you add in the padding for the frame, and account for the irregularity of the overall shape caused by the vertical height of this post, the left-margin of the the post is necessarily overhung.