I have managed to get a custom SWF embedded in Facebook wall posts. This can be done by adding special meta tags to the source code of the linked page. For example the respective source of a YouTube play page for this looks like the following:
<meta property="fb:app_id" content="87741124305">
<meta property="og:url" content="http://www.youtube.com/watch?v=qLkk3i4tMQw">
<meta property="og:title" content="Marco Huck vs Hugo Hernan Garay 10. Runde KO">
<meta property="og:description" content="...">
<meta property="og:type" content="video">
<meta property="og:image" content="http://i2.ytimg.com/vi/qLkk3i4tMQw/default.jpg">
<meta property="og:video" content="http://www.youtube.com/v/qLkk3i4tMQw?version=3&autohide=1">
<meta property="og:video:type" content="application/x-shockwave-flash">
<meta property="og:video:width" content="398">
<meta property="og:video:height" content="224">
<meta property="og:site_name" content="YouTube">
However, this apparently only works for Flash content.
Recently, Facebook has added support for HTML5 videos originating from YouTube (used for playing video on iPad.) It would be quite interesting to know whether this is limited to YouTube. Does anyone know whether it is possible for the rest of us to embed HTML(5) code into Facebook wall posts? If so, how can it be done?
Sorry, but there's currently no API to achieve what you suggest. HTML5 is a standard we're committed to though, so I wouldn't rule out this feature possibly being added in the future.